From 4acf5848843b1d1c69f10ab79a6da9176b90698a Mon Sep 17 00:00:00 2001
From: Samsondeen Dare <samsondeen.dare@hashicorp.com>
Date: Mon, 23 Feb 2026 10:43:39 +0100
Subject: [PATCH 01/11] Fill computed values of Nested type

---
 internal/command/test_test.go                 | 79 +++++++++++++++++++
 .../test/override_data_list_attribute/main.tf | 15 ++++
 .../main.tftest.hcl                           | 46 +++++++++++
 .../main.tf                                   | 15 ++++
 .../main.tftest.hcl                           | 46 +++++++++++
 internal/command/testing/test_provider.go     | 27 +++++++
 internal/moduletest/mocking/fill.go           | 77 ++++++++++--------
 7 files changed, 273 insertions(+), 32 deletions(-)
 create mode 100644 internal/command/testdata/test/override_data_list_attribute/main.tf
 create mode 100644 internal/command/testdata/test/override_data_list_attribute/main.tftest.hcl
 create mode 100644 internal/command/testdata/test/override_data_nested_list_attribute/main.tf
 create mode 100644 internal/command/testdata/test/override_data_nested_list_attribute/main.tftest.hcl

diff --git a/internal/command/test_test.go b/internal/command/test_test.go
index 92adc8573874..2960cc5c3859 100644
--- a/internal/command/test_test.go
+++ b/internal/command/test_test.go
@@ -5674,6 +5674,85 @@ func TestTest_TeardownOrder(t *testing.T) {
 	}
 }
 
+func TestTest_OverrideDataListAttribute(t *testing.T) {
+	tcs := map[string]struct {
+		dir  string
+		code int
+		desc string
+	}{
+		"plain_list_attribute": {
+			dir:  "override_data_list_attribute",
+			code: 0,
+			desc: "override_data with a computed cty.List(cty.Object) attribute",
+		},
+		"nested_list_attribute": {
+			dir:  "override_data_nested_list_attribute",
+			code: 0,
+			desc: "override_data with a computed NestedType NestingList attribute",
+		},
+	}
+
+	for name, tc := range tcs {
+		t.Run(name, func(t *testing.T) {
+			td := t.TempDir()
+			testCopyDir(t, testFixturePath(path.Join("test", tc.dir)), td)
+			t.Chdir(td)
+
+			provider := testing_command.NewProvider(nil)
+			providerSource, closeFn := newMockProviderSource(t, map[string][]string{
+				"test": {"1.0.0"},
+			})
+			defer closeFn()
+
+			streams, done := terminal.StreamsForTesting(t)
+			view := views.NewView(streams)
+			ui := new(cli.MockUi)
+
+			meta := Meta{
+				testingOverrides: metaOverridesForProvider(provider.Provider),
+				Ui:               ui,
+				View:             view,
+				Streams:          streams,
+				ProviderSource:   providerSource,
+			}
+
+			init := &InitCommand{
+				Meta: meta,
+			}
+
+			if code := init.Run(nil); code != 0 {
+				output := done(t)
+				t.Fatalf("expected init status code 0 but got %d: %s", code, output.All())
+			}
+
+			// Reset the streams for the next command.
+			streams, done = terminal.StreamsForTesting(t)
+			meta.Streams = streams
+			meta.View = views.NewView(streams)
+
+			c := &TestCommand{
+				Meta: meta,
+			}
+
+			code := c.Run([]string{"-no-color"})
+			output := done(t)
+
+			if code != tc.code {
+				t.Errorf("expected status code %d but got %d:\n\n%s", tc.code, code, output.All())
+			}
+
+			if tc.code == 0 {
+				if !strings.Contains(output.Stdout(), "1 passed, 0 failed.") {
+					t.Errorf("expected passing test output but got:\n\nstdout:\n%s\nstderr:\n%s", output.Stdout(), output.Stderr())
+				}
+				if output.Stderr() != "" {
+					t.Errorf("unexpected stderr output:\n%s", output.Stderr())
+				}
+			}
+		})
+	}
+}
+
 // testModuleInline takes a map of path -> config strings and yields a config
 // structure with those files loaded from disk
 func testModuleInline(t *testing.T, sources map[string]string) (*configs.Config, string, func()) {
diff --git a/internal/command/testdata/test/override_data_list_attribute/main.tf b/internal/command/testdata/test/override_data_list_attribute/main.tf
new file mode 100644
index 000000000000..f9d19bfd7435
--- /dev/null
+++ b/internal/command/testdata/test/override_data_list_attribute/main.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    test = {
+      source = "hashicorp/test"
+    }
+  }
+}
+
+data "test_data_source" "datasource" {
+  id = "resource"
+}
+
+output "list_value" {
+  value = data.test_data_source.datasource.list_value
+}
diff --git a/internal/command/testdata/test/override_data_list_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_list_attribute/main.tftest.hcl
new file mode 100644
index 000000000000..597e98ac4f9f
--- /dev/null
+++ b/internal/command/testdata/test/override_data_list_attribute/main.tftest.hcl
@@ -0,0 +1,46 @@
+provider "test" {}
+
+override_data {
+  target = data.test_data_source.datasource
+  values = {
+    list_value = [
+      {
+        name  = "first"
+        value = "one"
+      },
+      {
+        name  = "second"
+        value = "two"
+      },
+    ]
+  }
+}
+
+run "test_override_data_list_attribute" {
+  command = plan
+
+  assert {
+    condition     = length(data.test_data_source.datasource.list_value) == 2
+    error_message = "Expected list_value to have 2 elements, got ${length(data.test_data_source.datasource.list_value)}"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.list_value[0].name == "first"
+    error_message = "Expected first element name to be 'first'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.list_value[0].value == "one"
+    error_message = "Expected first element value to be 'one'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.list_value[1].name == "second"
+    error_message = "Expected second element name to be 'second'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.list_value[1].value == "two"
+    error_message = "Expected second element value to be 'two'"
+  }
+}
diff --git a/internal/command/testdata/test/override_data_nested_list_attribute/main.tf b/internal/command/testdata/test/override_data_nested_list_attribute/main.tf
new file mode 100644
index 000000000000..52cfeb5fa44a
--- /dev/null
+++ b/internal/command/testdata/test/override_data_nested_list_attribute/main.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    test = {
+      source = "hashicorp/test"
+    }
+  }
+}
+
+data "test_data_source" "datasource" {
+  id = "resource"
+}
+
+output "nested_list_value" {
+  value = data.test_data_source.datasource.nested_list_value
+}
diff --git a/internal/command/testdata/test/override_data_nested_list_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_nested_list_attribute/main.tftest.hcl
new file mode 100644
index 000000000000..fe5a85019600
--- /dev/null
+++ b/internal/command/testdata/test/override_data_nested_list_attribute/main.tftest.hcl
@@ -0,0 +1,46 @@
+provider "test" {}
+
+override_data {
+  target = data.test_data_source.datasource
+  values = {
+    nested_list_value = [
+      {
+        name  = "first"
+        value = "one"
+      },
+      {
+        name  = "second"
+        value = "two"
+      },
+    ]
+  }
+}
+
+run "test_override_data_nested_list_attribute" {
+  command = plan
+
+  assert {
+    condition     = length(data.test_data_source.datasource.nested_list_value) == 2
+    error_message = "Expected nested_list_value to have 2 elements, got ${length(data.test_data_source.datasource.nested_list_value)}"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.nested_list_value[0].name == "first"
+    error_message = "Expected first element name to be 'first'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.nested_list_value[0].value == "one"
+    error_message = "Expected first element value to be 'one'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.nested_list_value[1].name == "second"
+    error_message = "Expected second element name to be 'second'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.nested_list_value[1].value == "two"
+    error_message = "Expected second element value to be 'two'"
+  }
+}
diff --git a/internal/command/testing/test_provider.go b/internal/command/testing/test_provider.go
index f1a8e95f9751..bd2ed41ac845 100644
--- a/internal/command/testing/test_provider.go
+++ b/internal/command/testing/test_provider.go
@@ -53,6 +53,33 @@ var (
 						"value":      {Type: cty.String, Computed: true},
 						"write_only": {Type: cty.String, Optional: true, WriteOnly: true},
 
+						// list_value is a computed attribute of list-of-objects
+						// type, used to test that override_data can correctly
+						// override list-type attributes (see GitHub issue #37939).
+						"list_value": {
+							Type: cty.List(cty.Object(map[string]cty.Type{
+								"name":  cty.String,
+								"value": cty.String,
+							})),
+							Computed: true,
+						},
+
+						// nested_list_value is a computed attribute using NestedType
+						// with NestingList, used to test that override_data can correctly
+						// override nested list-type attributes (see GitHub issue #37939).
+						// Many real-world providers (e.g. Databricks) define list-of-objects
+						// attributes this way rather than using a plain cty.List type.
+						"nested_list_value": {
+							Computed: true,
+							NestedType: &configschema.Object{
+								Nesting: configschema.NestingList,
+								Attributes: map[string]*configschema.Attribute{
+									"name":  {Type: cty.String, Optional: true},
+									"value": {Type: cty.String, Optional: true},
+								},
+							},
+						},
+
 						// We never actually reference these values from a data
 						// source, but we have tests that use the same cty.Value
 						// to represent a test_resource and a test_data_source
diff --git a/internal/moduletest/mocking/fill.go b/internal/moduletest/mocking/fill.go
index f8643aa10b02..e3431eeeb978 100644
--- a/internal/moduletest/mocking/fill.go
+++ b/internal/moduletest/mocking/fill.go
@@ -24,43 +24,13 @@ func FillAttribute(in cty.Value, attribute *configschema.Attribute) (cty.Value,
 func fillAttribute(in cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
 	if attribute.NestedType != nil {
 
-		// Then the in value must be an object.
-		if !in.Type().IsObjectType() {
-			return cty.NilVal, path.NewErrorf("incompatible types; expected object type, found %s", in.Type().FriendlyName())
-		}
-
 		switch attribute.NestedType.Nesting {
 		case configschema.NestingSingle, configschema.NestingGroup:
-			var names []string
-			for name := range attribute.NestedType.Attributes {
-				names = append(names, name)
-			}
-			if len(names) == 0 {
-				return cty.EmptyObjectVal, nil
-			}
-
-			// Make the order we iterate through the attributes deterministic. We
-			// are generating random strings in here so it's worth making the
-			// operation repeatable.
-			sort.Strings(names)
-
-			children := make(map[string]cty.Value)
-			for _, name := range names {
-				if in.Type().HasAttribute(name) {
-					child, err := fillAttribute(in.GetAttr(name), attribute.NestedType.Attributes[name], path.GetAttr(name))
-					if err != nil {
-						return cty.NilVal, err
-					}
-					children[name] = child
-					continue
-				}
-				children[name] = GenerateValueForAttribute(attribute.NestedType.Attributes[name])
-			}
-			return cty.ObjectVal(children), nil
+			return fillObject(in, attribute, path)
 		case configschema.NestingSet:
 			return cty.SetValEmpty(attribute.ImpliedType().ElementType()), nil
 		case configschema.NestingList:
-			return cty.ListValEmpty(attribute.ImpliedType().ElementType()), nil
+			return fillIterable(in, attribute, path)
 		case configschema.NestingMap:
 			return cty.MapValEmpty(attribute.ImpliedType().ElementType()), nil
 		default:
@@ -71,6 +41,49 @@ func fillAttribute(in cty.Value, attribute *configschema.Attribute, path cty.Pat
 	return fillType(in, attribute.Type, path)
 }
 
+func fillObject(in cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
+	// Then the in value must be an object.
+	if !in.Type().IsObjectType() {
+		return cty.NilVal, path.NewErrorf("incompatible types; expected object type, found %s", in.Type().FriendlyName())
+	}
+
+	var names []string
+	for name := range attribute.NestedType.Attributes {
+		names = append(names, name)
+	}
+	if len(names) == 0 {
+		return cty.EmptyObjectVal, nil
+	}
+
+	// Make the order we iterate through the attributes deterministic. We
+	// are generating random strings in here so it's worth making the
+	// operation repeatable.
+	sort.Strings(names)
+
+	children := make(map[string]cty.Value)
+	for _, name := range names {
+		if in.Type().HasAttribute(name) {
+			child, err := fillAttribute(in.GetAttr(name), attribute.NestedType.Attributes[name], path.GetAttr(name))
+			if err != nil {
+				return cty.NilVal, err
+			}
+			children[name] = child
+			continue
+		}
+		children[name] = GenerateValueForAttribute(attribute.NestedType.Attributes[name])
+	}
+	return cty.ObjectVal(children), nil
+}
+
+func fillIterable(in cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
+	ty := attribute.NestedType.ConfigType()
+	out, err := fillType(in, ty, path)
+	if err != nil {
+		return cty.NilVal, err
+	}
+	return out, err
+}
+
 // FillType makes the input value match the target type by adding attributes
 // directly to it or to any nested objects. Essentially, this is a "safe"
 // conversion between two objects.

From 08d5e4d07739c816ee03bd20a87bdf8a1ea71966 Mon Sep 17 00:00:00 2001
From: Samsondeen Dare <samsondeen.dare@hashicorp.com>
Date: Mon, 23 Feb 2026 11:26:17 +0100
Subject: [PATCH 02/11] more tests

---
 internal/command/test_test.go                 |  24 +-
 .../test/override_data_list_attribute/main.tf |   4 +-
 .../main.tftest.hcl                           |  23 +-
 .../test/override_data_map_attribute/main.tf  |  15 +
 .../main.tftest.hcl                           |  31 +
 .../main.tf                                   |   4 +-
 .../main.tftest.hcl                           |  23 +-
 .../main.tf                                   |  15 +
 .../main.tftest.hcl                           |  32 +
 .../main.tf                                   |  15 +
 .../main.tftest.hcl                           |  31 +
 .../test/override_data_set_attribute/main.tf  |  15 +
 .../main.tftest.hcl                           |  31 +
 internal/command/testing/test_provider.go     |  71 +-
 internal/moduletest/mocking/fill.go           |  10 +-
 internal/moduletest/mocking/fill_test.go      | 859 ++++++++++++++++++
 internal/moduletest/mocking/values_test.go    |   3 +
 17 files changed, 1147 insertions(+), 59 deletions(-)
 create mode 100644 internal/command/testdata/test/override_data_map_attribute/main.tf
 create mode 100644 internal/command/testdata/test/override_data_map_attribute/main.tftest.hcl
 create mode 100644 internal/command/testdata/test/override_data_nested_map_attribute/main.tf
 create mode 100644 internal/command/testdata/test/override_data_nested_map_attribute/main.tftest.hcl
 create mode 100644 internal/command/testdata/test/override_data_nested_set_attribute/main.tf
 create mode 100644 internal/command/testdata/test/override_data_nested_set_attribute/main.tftest.hcl
 create mode 100644 internal/command/testdata/test/override_data_set_attribute/main.tf
 create mode 100644 internal/command/testdata/test/override_data_set_attribute/main.tftest.hcl

diff --git a/internal/command/test_test.go b/internal/command/test_test.go
index 2960cc5c3859..a27a34dc96fc 100644
--- a/internal/command/test_test.go
+++ b/internal/command/test_test.go
@@ -5674,13 +5674,13 @@ func TestTest_TeardownOrder(t *testing.T) {
 	}
 }
 
-func TestTest_OverrideDataListAttribute(t *testing.T) {
+func TestTest_OverrideData(t *testing.T) {
 	tcs := map[string]struct {
 		dir  string
 		code int
 		desc string
 	}{
-		"plain_list_attribute": {
+		"list_attribute": {
 			dir:  "override_data_list_attribute",
 			code: 0,
 			desc: "override_data with a computed cty.List(cty.Object) attribute",
@@ -5690,6 +5690,26 @@ func TestTest_OverrideDataListAttribute(t *testing.T) {
 			code: 0,
 			desc: "override_data with a computed NestedType NestingList attribute",
 		},
+		"set_attribute": {
+			dir:  "override_data_set_attribute",
+			code: 0,
+			desc: "override_data with a computed cty.Set(cty.Object) attribute",
+		},
+		"nested_set_attribute": {
+			dir:  "override_data_nested_set_attribute",
+			code: 0,
+			desc: "override_data with a computed NestedType NestingSet attribute",
+		},
+		"map_attribute": {
+			dir:  "override_data_map_attribute",
+			code: 0,
+			desc: "override_data with a computed cty.Map(cty.Object) attribute",
+		},
+		"nested_map_attribute": {
+			dir:  "override_data_nested_map_attribute",
+			code: 0,
+			desc: "override_data with a computed NestedType NestingMap attribute",
+		},
 	}
 
 	for name, tc := range tcs {
diff --git a/internal/command/testdata/test/override_data_list_attribute/main.tf b/internal/command/testdata/test/override_data_list_attribute/main.tf
index f9d19bfd7435..00330c09aea6 100644
--- a/internal/command/testdata/test/override_data_list_attribute/main.tf
+++ b/internal/command/testdata/test/override_data_list_attribute/main.tf
@@ -6,10 +6,10 @@ terraform {
   }
 }
 
-data "test_data_source" "datasource" {
+data "test_complex_data_source" "datasource" {
   id = "resource"
 }
 
 output "list_value" {
-  value = data.test_data_source.datasource.list_value
+  value = data.test_complex_data_source.datasource.list_value
 }
diff --git a/internal/command/testdata/test/override_data_list_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_list_attribute/main.tftest.hcl
index 597e98ac4f9f..a0518942621c 100644
--- a/internal/command/testdata/test/override_data_list_attribute/main.tftest.hcl
+++ b/internal/command/testdata/test/override_data_list_attribute/main.tftest.hcl
@@ -1,7 +1,7 @@
 provider "test" {}
 
 override_data {
-  target = data.test_data_source.datasource
+  target = data.test_complex_data_source.datasource
   values = {
     list_value = [
       {
@@ -20,27 +20,12 @@ run "test_override_data_list_attribute" {
   command = plan
 
   assert {
-    condition     = length(data.test_data_source.datasource.list_value) == 2
-    error_message = "Expected list_value to have 2 elements, got ${length(data.test_data_source.datasource.list_value)}"
+    condition     = length(data.test_complex_data_source.datasource.list_value) == 2
+    error_message = "Expected list_value to have 2 elements, got ${length(data.test_complex_data_source.datasource.list_value)}"
   }
 
   assert {
-    condition     = data.test_data_source.datasource.list_value[0].name == "first"
+    condition     = data.test_complex_data_source.datasource.list_value[0].name == "first"
     error_message = "Expected first element name to be 'first'"
   }
-
-  assert {
-    condition     = data.test_data_source.datasource.list_value[0].value == "one"
-    error_message = "Expected first element value to be 'one'"
-  }
-
-  assert {
-    condition     = data.test_data_source.datasource.list_value[1].name == "second"
-    error_message = "Expected second element name to be 'second'"
-  }
-
-  assert {
-    condition     = data.test_data_source.datasource.list_value[1].value == "two"
-    error_message = "Expected second element value to be 'two'"
-  }
 }
diff --git a/internal/command/testdata/test/override_data_map_attribute/main.tf b/internal/command/testdata/test/override_data_map_attribute/main.tf
new file mode 100644
index 000000000000..520c9e1de8bf
--- /dev/null
+++ b/internal/command/testdata/test/override_data_map_attribute/main.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    test = {
+      source = "hashicorp/test"
+    }
+  }
+}
+
+data "test_complex_data_source" "datasource" {
+  id = "resource"
+}
+
+output "map_value" {
+  value = data.test_complex_data_source.datasource.map_value
+}
diff --git a/internal/command/testdata/test/override_data_map_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_map_attribute/main.tftest.hcl
new file mode 100644
index 000000000000..02d7b8311751
--- /dev/null
+++ b/internal/command/testdata/test/override_data_map_attribute/main.tftest.hcl
@@ -0,0 +1,31 @@
+provider "test" {}
+
+override_data {
+  target = data.test_complex_data_source.datasource
+  values = {
+    map_value = {
+      "key1" = {
+        name  = "first"
+        value = "one"
+      }
+      "key2" = {
+        name  = "second"
+        value = "two"
+      }
+    }
+  }
+}
+
+run "test_override_data_map_attribute" {
+  command = plan
+
+  assert {
+    condition     = length(data.test_complex_data_source.datasource.map_value) == 2
+    error_message = "Expected map_value to have 2 elements, got ${length(data.test_complex_data_source.datasource.map_value)}"
+  }
+
+  assert {
+    condition     = data.test_complex_data_source.datasource.map_value["key1"].name == "first"
+    error_message = "Expected key1 name to be 'first'"
+  }
+}
diff --git a/internal/command/testdata/test/override_data_nested_list_attribute/main.tf b/internal/command/testdata/test/override_data_nested_list_attribute/main.tf
index 52cfeb5fa44a..ea41b36bc2e2 100644
--- a/internal/command/testdata/test/override_data_nested_list_attribute/main.tf
+++ b/internal/command/testdata/test/override_data_nested_list_attribute/main.tf
@@ -6,10 +6,10 @@ terraform {
   }
 }
 
-data "test_data_source" "datasource" {
+data "test_complex_data_source" "datasource" {
   id = "resource"
 }
 
 output "nested_list_value" {
-  value = data.test_data_source.datasource.nested_list_value
+  value = data.test_complex_data_source.datasource.nested_list_value
 }
diff --git a/internal/command/testdata/test/override_data_nested_list_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_nested_list_attribute/main.tftest.hcl
index fe5a85019600..a02327798074 100644
--- a/internal/command/testdata/test/override_data_nested_list_attribute/main.tftest.hcl
+++ b/internal/command/testdata/test/override_data_nested_list_attribute/main.tftest.hcl
@@ -1,7 +1,7 @@
 provider "test" {}
 
 override_data {
-  target = data.test_data_source.datasource
+  target = data.test_complex_data_source.datasource
   values = {
     nested_list_value = [
       {
@@ -20,27 +20,12 @@ run "test_override_data_nested_list_attribute" {
   command = plan
 
   assert {
-    condition     = length(data.test_data_source.datasource.nested_list_value) == 2
-    error_message = "Expected nested_list_value to have 2 elements, got ${length(data.test_data_source.datasource.nested_list_value)}"
+    condition     = length(data.test_complex_data_source.datasource.nested_list_value) == 2
+    error_message = "Expected nested_list_value to have 2 elements, got ${length(data.test_complex_data_source.datasource.nested_list_value)}"
   }
 
   assert {
-    condition     = data.test_data_source.datasource.nested_list_value[0].name == "first"
+    condition     = data.test_complex_data_source.datasource.nested_list_value[0].name == "first"
     error_message = "Expected first element name to be 'first'"
   }
-
-  assert {
-    condition     = data.test_data_source.datasource.nested_list_value[0].value == "one"
-    error_message = "Expected first element value to be 'one'"
-  }
-
-  assert {
-    condition     = data.test_data_source.datasource.nested_list_value[1].name == "second"
-    error_message = "Expected second element name to be 'second'"
-  }
-
-  assert {
-    condition     = data.test_data_source.datasource.nested_list_value[1].value == "two"
-    error_message = "Expected second element value to be 'two'"
-  }
 }
diff --git a/internal/command/testdata/test/override_data_nested_map_attribute/main.tf b/internal/command/testdata/test/override_data_nested_map_attribute/main.tf
new file mode 100644
index 000000000000..0df44632df41
--- /dev/null
+++ b/internal/command/testdata/test/override_data_nested_map_attribute/main.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    test = {
+      source = "hashicorp/test"
+    }
+  }
+}
+
+data "test_complex_data_source" "datasource" {
+  id = "resource"
+}
+
+output "nested_map_value" {
+  value = data.test_complex_data_source.datasource.nested_map_value
+}
diff --git a/internal/command/testdata/test/override_data_nested_map_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_nested_map_attribute/main.tftest.hcl
new file mode 100644
index 000000000000..844f18adf686
--- /dev/null
+++ b/internal/command/testdata/test/override_data_nested_map_attribute/main.tftest.hcl
@@ -0,0 +1,32 @@
+provider "test" {}
+
+override_data {
+  target = data.test_complex_data_source.datasource
+  values = {
+    nested_map_value = {
+      "key1" = {
+        name  = "first"
+        value = "one"
+      }
+      "key2" = {
+        name  = "second"
+        value = "two"
+      }
+    }
+  }
+}
+
+run "test_override_data_nested_map_attribute" {
+  command = plan
+
+  assert {
+    condition     = length(data.test_complex_data_source.datasource.nested_map_value) == 2
+    error_message = "Expected nested_map_value to have 2 elements, got ${length(data.test_complex_data_source.datasource.nested_map_value)}"
+  }
+
+  assert {
+    condition     = data.test_complex_data_source.datasource.nested_map_value["key1"].name == "first"
+    error_message = "Expected key1 name to be 'first'"
+  }
+
+}
diff --git a/internal/command/testdata/test/override_data_nested_set_attribute/main.tf b/internal/command/testdata/test/override_data_nested_set_attribute/main.tf
new file mode 100644
index 000000000000..5174b78b954f
--- /dev/null
+++ b/internal/command/testdata/test/override_data_nested_set_attribute/main.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    test = {
+      source = "hashicorp/test"
+    }
+  }
+}
+
+data "test_complex_data_source" "datasource" {
+  id = "resource"
+}
+
+output "nested_set_value" {
+  value = data.test_complex_data_source.datasource.nested_set_value
+}
diff --git a/internal/command/testdata/test/override_data_nested_set_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_nested_set_attribute/main.tftest.hcl
new file mode 100644
index 000000000000..ef4dc0638749
--- /dev/null
+++ b/internal/command/testdata/test/override_data_nested_set_attribute/main.tftest.hcl
@@ -0,0 +1,31 @@
+provider "test" {}
+
+override_data {
+  target = data.test_complex_data_source.datasource
+  values = {
+    nested_set_value = [
+      {
+        name  = "first"
+        value = "one"
+      },
+      {
+        name  = "second"
+        value = "two"
+      },
+    ]
+  }
+}
+
+run "test_override_data_nested_set_attribute" {
+  command = plan
+
+  assert {
+    condition     = length(data.test_complex_data_source.datasource.nested_set_value) == 2
+    error_message = "Expected nested_set_value to have 2 elements, got ${length(data.test_complex_data_source.datasource.nested_set_value)}"
+  }
+
+  assert {
+    condition     = contains([for item in data.test_complex_data_source.datasource.nested_set_value : item.name], "first")
+    error_message = "Expected nested_set_value to contain an element with name 'first'"
+  }
+}
diff --git a/internal/command/testdata/test/override_data_set_attribute/main.tf b/internal/command/testdata/test/override_data_set_attribute/main.tf
new file mode 100644
index 000000000000..bbea87bd356d
--- /dev/null
+++ b/internal/command/testdata/test/override_data_set_attribute/main.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    test = {
+      source = "hashicorp/test"
+    }
+  }
+}
+
+data "test_complex_data_source" "datasource" {
+  id = "resource"
+}
+
+output "set_value" {
+  value = data.test_complex_data_source.datasource.set_value
+}
diff --git a/internal/command/testdata/test/override_data_set_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_set_attribute/main.tftest.hcl
new file mode 100644
index 000000000000..cac115aba854
--- /dev/null
+++ b/internal/command/testdata/test/override_data_set_attribute/main.tftest.hcl
@@ -0,0 +1,31 @@
+provider "test" {}
+
+override_data {
+  target = data.test_complex_data_source.datasource
+  values = {
+    set_value = [
+      {
+        name  = "first"
+        value = "one"
+      },
+      {
+        name  = "second"
+        value = "two"
+      },
+    ]
+  }
+}
+
+run "test_override_data_set_attribute" {
+  command = plan
+
+  assert {
+    condition     = length(data.test_complex_data_source.datasource.set_value) == 2
+    error_message = "Expected set_value to have 2 elements, got ${length(data.test_complex_data_source.datasource.set_value)}"
+  }
+
+  assert {
+    condition     = contains([for item in data.test_complex_data_source.datasource.set_value : item.name], "first")
+    error_message = "Expected set_value to contain an element with name 'first'"
+  }
+}
diff --git a/internal/command/testing/test_provider.go b/internal/command/testing/test_provider.go
index bd2ed41ac845..e40bda8ca231 100644
--- a/internal/command/testing/test_provider.go
+++ b/internal/command/testing/test_provider.go
@@ -53,24 +53,57 @@ var (
 						"value":      {Type: cty.String, Computed: true},
 						"write_only": {Type: cty.String, Optional: true, WriteOnly: true},
 
-						// list_value is a computed attribute of list-of-objects
-						// type, used to test that override_data can correctly
-						// override list-type attributes (see GitHub issue #37939).
+						// We never actually reference these values from a data
+						// source, but we have tests that use the same cty.Value
+						// to represent a test_resource and a test_data_source
+						// so the schemas have to match.
+
+						"interrupt_count":      {Type: cty.Number, Computed: true},
+						"destroy_fail":         {Type: cty.Bool, Computed: true},
+						"create_wait_seconds":  {Type: cty.Number, Computed: true},
+						"destroy_wait_seconds": {Type: cty.Number, Computed: true},
+						"defer":                {Type: cty.Bool, Computed: true},
+					},
+				},
+			},
+			"test_complex_data_source": {
+				Body: &configschema.Block{
+					Attributes: map[string]*configschema.Attribute{
+						"id":         {Type: cty.String, Required: true},
+						"value":      {Type: cty.String, Computed: true},
+						"write_only": {Type: cty.String, Optional: true, WriteOnly: true},
+
 						"list_value": {
 							Type: cty.List(cty.Object(map[string]cty.Type{
 								"name":  cty.String,
 								"value": cty.String,
 							})),
 							Computed: true,
+							Optional: true,
+						},
+						"set_value": {
+							Type: cty.Set(cty.Object(map[string]cty.Type{
+								"name":  cty.String,
+								"value": cty.String,
+							})),
+							Computed: true,
+							Optional: true,
+						},
+						"map_value": {
+							Type: cty.Map(cty.Object(map[string]cty.Type{
+								"name":  cty.String,
+								"value": cty.String,
+							})),
+							Computed: true,
+							Optional: true,
 						},
 
-						// nested_list_value is a computed attribute using NestedType
-						// with NestingList, used to test that override_data can correctly
-						// override nested list-type attributes (see GitHub issue #37939).
-						// Many real-world providers (e.g. Databricks) define list-of-objects
-						// attributes this way rather than using a plain cty.List type.
+						// The below nested_* attributes represent config supporting
+						// attributes as nested blocks, where terraform interprets the nested blocks
+						// as attributes within the schema
 						"nested_list_value": {
 							Computed: true,
+							Optional: true,
 							NestedType: &configschema.Object{
 								Nesting: configschema.NestingList,
 								Attributes: map[string]*configschema.Attribute{
@@ -79,6 +112,28 @@ var (
 								},
 							},
 						},
+						"nested_set_value": {
+							Computed: true,
+							Optional: true,
+							NestedType: &configschema.Object{
+								Nesting: configschema.NestingSet,
+								Attributes: map[string]*configschema.Attribute{
+									"name":  {Type: cty.String, Optional: true},
+									"value": {Type: cty.String, Optional: true},
+								},
+							},
+						},
+						"nested_map_value": {
+							Computed: true,
+							Optional: true,
+							NestedType: &configschema.Object{
+								Nesting: configschema.NestingMap,
+								Attributes: map[string]*configschema.Attribute{
+									"name":  {Type: cty.String, Optional: true},
+									"value": {Type: cty.String, Optional: true},
+								},
+							},
+						},
 
 						// We never actually reference these values from a data
 						// source, but we have tests that use the same cty.Value
diff --git a/internal/moduletest/mocking/fill.go b/internal/moduletest/mocking/fill.go
index e3431eeeb978..676bf815893f 100644
--- a/internal/moduletest/mocking/fill.go
+++ b/internal/moduletest/mocking/fill.go
@@ -28,11 +28,11 @@ func fillAttribute(in cty.Value, attribute *configschema.Attribute, path cty.Pat
 		case configschema.NestingSingle, configschema.NestingGroup:
 			return fillObject(in, attribute, path)
 		case configschema.NestingSet:
-			return cty.SetValEmpty(attribute.ImpliedType().ElementType()), nil
+			return fillIterable(in, attribute, path)
 		case configschema.NestingList:
 			return fillIterable(in, attribute, path)
 		case configschema.NestingMap:
-			return cty.MapValEmpty(attribute.ImpliedType().ElementType()), nil
+			return fillIterable(in, attribute, path)
 		default:
 			panic(fmt.Errorf("unknown nesting mode: %d", attribute.NestedType.Nesting))
 		}
@@ -77,11 +77,7 @@ func fillObject(in cty.Value, attribute *configschema.Attribute, path cty.Path)
 
 func fillIterable(in cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
 	ty := attribute.NestedType.ConfigType()
-	out, err := fillType(in, ty, path)
-	if err != nil {
-		return cty.NilVal, err
-	}
-	return out, err
+	return fillType(in, ty, path)
 }
 
 // FillType makes the input value match the target type by adding attributes
diff --git a/internal/moduletest/mocking/fill_test.go b/internal/moduletest/mocking/fill_test.go
index 008c0fbe804c..6701eb6e66d4 100644
--- a/internal/moduletest/mocking/fill_test.go
+++ b/internal/moduletest/mocking/fill_test.go
@@ -8,8 +8,867 @@ import (
 	"testing"
 
 	"github.com/zclconf/go-cty/cty"
+
+	"github.com/hashicorp/terraform/internal/configs/configschema"
 )
 
+// TestFillAttribute tests the FillAttribute function which handles
+// configschema.Attribute values including NestedType attributes with various
+// nesting modes (NestingSingle, NestingGroup, NestingList, NestingSet, NestingMap).
+func TestFillAttribute(t *testing.T) {
+	tcs := map[string]struct {
+		in        cty.Value
+		attribute *configschema.Attribute
+		expected  cty.Value
+	}{
+		// =====================================================================
+		// Plain attributes (no NestedType) - falls through to fillType
+		// =====================================================================
+
+		"plain_string_attribute": {
+			in: cty.StringVal("hello"),
+			attribute: &configschema.Attribute{
+				Type: cty.String,
+			},
+			expected: cty.StringVal("hello"),
+		},
+
+		"plain_number_to_string_conversion": {
+			in: cty.NumberIntVal(42),
+			attribute: &configschema.Attribute{
+				Type: cty.String,
+			},
+			expected: cty.StringVal("42"),
+		},
+
+		"plain_list_of_objects_attribute": {
+			// When the attribute uses Type (not NestedType), it goes through
+			// fillType directly. This is the "plain" list-of-objects case
+			// that already worked before the fix.
+			in: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name": cty.StringVal("first"),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				Type: cty.List(cty.Object(map[string]cty.Type{
+					"name":  cty.String,
+					"value": cty.String,
+				})),
+			},
+			expected: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"value": cty.StringVal("ssnk9qhr"),
+				}),
+			}),
+		},
+
+		// =====================================================================
+		// NestingSingle
+		// =====================================================================
+
+		"nesting_single_partial_attrs": {
+			in: cty.ObjectVal(map[string]cty.Value{
+				"name": cty.StringVal("hello"),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingSingle,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String},
+						"value": {Type: cty.String},
+					},
+				},
+			},
+			expected: cty.ObjectVal(map[string]cty.Value{
+				"name":  cty.StringVal("hello"),
+				"value": cty.StringVal("ssnk9qhr"),
+			}),
+		},
+
+		"nesting_single_all_attrs_present": {
+			in: cty.ObjectVal(map[string]cty.Value{
+				"name":  cty.StringVal("hello"),
+				"value": cty.StringVal("world"),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingSingle,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String},
+						"value": {Type: cty.String},
+					},
+				},
+			},
+			expected: cty.ObjectVal(map[string]cty.Value{
+				"name":  cty.StringVal("hello"),
+				"value": cty.StringVal("world"),
+			}),
+		},
+
+		"nesting_single_empty_schema": {
+			in: cty.EmptyObjectVal,
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting:    configschema.NestingSingle,
+					Attributes: map[string]*configschema.Attribute{},
+				},
+			},
+			expected: cty.EmptyObjectVal,
+		},
+
+		"nesting_single_extra_attrs_in_input_dropped": {
+			// Input has attributes not in the schema; they should be dropped.
+			in: cty.ObjectVal(map[string]cty.Value{
+				"name":  cty.StringVal("hello"),
+				"extra": cty.StringVal("should be dropped"),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingSingle,
+					Attributes: map[string]*configschema.Attribute{
+						"name": {Type: cty.String},
+					},
+				},
+			},
+			expected: cty.ObjectVal(map[string]cty.Value{
+				"name": cty.StringVal("hello"),
+			}),
+		},
+
+		// =====================================================================
+		// NestingGroup (behaves same as NestingSingle for fill purposes)
+		// =====================================================================
+
+		"nesting_group_partial_attrs": {
+			in: cty.ObjectVal(map[string]cty.Value{
+				"name": cty.StringVal("hello"),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingGroup,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String},
+						"value": {Type: cty.String},
+					},
+				},
+			},
+			expected: cty.ObjectVal(map[string]cty.Value{
+				"name":  cty.StringVal("hello"),
+				"value": cty.StringVal("ssnk9qhr"),
+			}),
+		},
+
+		// =====================================================================
+		// NestingList - the main bug fix (GitHub issue #37939)
+		// =====================================================================
+
+		"nesting_list_tuple_input_partial_attrs": {
+			// This is the core bug scenario: HCL produces a tuple when parsing
+			// list literals in override_data values. The old code would fail
+			// or return empty for this input.
+			in: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name": cty.StringVal("first"),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String},
+						"value": {Type: cty.String},
+					},
+				},
+			},
+			expected: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"value": cty.StringVal("ssnk9qhr"),
+				}),
+			}),
+		},
+
+		"nesting_list_list_input_partial_attrs": {
+			// Input is already a proper list (not a tuple).
+			in: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name": cty.StringVal("first"),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String},
+						"value": {Type: cty.String},
+					},
+				},
+			},
+			expected: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"value": cty.StringVal("ssnk9qhr"),
+				}),
+			}),
+		},
+
+		"nesting_list_multiple_elements": {
+			in: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name": cty.StringVal("first"),
+				}),
+				cty.ObjectVal(map[string]cty.Value{
+					"name": cty.StringVal("second"),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String},
+						"value": {Type: cty.String},
+					},
+				},
+			},
+			expected: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"value": cty.StringVal("ssnk9qhr"),
+				}),
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("second"),
+					"value": cty.StringVal("amyllmyg"),
+				}),
+			}),
+		},
+
+		"nesting_list_all_attrs_present": {
+			in: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"value": cty.StringVal("one"),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String},
+						"value": {Type: cty.String},
+					},
+				},
+			},
+			expected: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"value": cty.StringVal("one"),
+				}),
+			}),
+		},
+
+		"nesting_list_extra_attrs_in_elements_dropped": {
+			// Input objects have attributes not defined in the schema.
+			in: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"value": cty.StringVal("one"),
+					"extra": cty.StringVal("should be dropped"),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String},
+						"value": {Type: cty.String},
+					},
+				},
+			},
+			expected: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"value": cty.StringVal("one"),
+				}),
+			}),
+		},
+
+		"nesting_list_set_input": {
+			// A set can also be converted to a list.
+			in: cty.SetVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"value": cty.StringVal("one"),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String},
+						"value": {Type: cty.String},
+					},
+				},
+			},
+			expected: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"value": cty.StringVal("one"),
+				}),
+			}),
+		},
+
+		"nesting_list_element_type_conversion": {
+			// Elements have attributes that need type conversion (number→string).
+			in: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"count": cty.NumberIntVal(42),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String},
+						"count": {Type: cty.String},
+					},
+				},
+			},
+			expected: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"count": cty.StringVal("42"),
+				}),
+			}),
+		},
+
+		"nesting_list_three_attrs_partial": {
+			// Object schema with 3 attributes, only 1 provided.
+			// Attributes are filled in alphabetical order: "alpha" (provided),
+			// "beta" (generated=ssnk9qhr), "gamma" (generated=amyllmyg).
+			in: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"alpha": cty.StringVal("provided"),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"alpha": {Type: cty.String},
+						"beta":  {Type: cty.String},
+						"gamma": {Type: cty.String},
+					},
+				},
+			},
+			expected: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"alpha": cty.StringVal("provided"),
+					"beta":  cty.StringVal("ssnk9qhr"),
+					"gamma": cty.StringVal("amyllmyg"),
+				}),
+			}),
+		},
+
+		// =====================================================================
+		// NestingSet - BUG: current implementation ignores input and always
+		// returns an empty set. These tests document the CURRENT behavior.
+		// If NestingSet is fixed to use fillIterable (like NestingList),
+		// these tests should be updated to expect filled values.
+		// =====================================================================
+
+		"nesting_set_returns_empty_ignoring_input": {
+			// BUG: The current implementation returns an empty set regardless
+			// of the input. Override values for NestingSet attributes are
+			// silently dropped, just like the original NestingList bug.
+			in: cty.SetVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"value": cty.StringVal("one"),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingSet,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String},
+						"value": {Type: cty.String},
+					},
+				},
+			},
+			// BUG: should be cty.SetVal with the input values preserved,
+			// but current code returns empty.
+			expected: cty.SetValEmpty(cty.Object(map[string]cty.Type{
+				"name":  cty.String,
+				"value": cty.String,
+			})),
+		},
+
+		// =====================================================================
+		// NestingMap - BUG: current implementation ignores input and always
+		// returns an empty map. These tests document the CURRENT behavior.
+		// If NestingMap is fixed to use fillIterable (like NestingList),
+		// these tests should be updated to expect filled values.
+		// =====================================================================
+
+		"nesting_map_returns_empty_ignoring_input": {
+			// BUG: The current implementation returns an empty map regardless
+			// of the input. Override values for NestingMap attributes are
+			// silently dropped, just like the original NestingList bug.
+			in: cty.MapVal(map[string]cty.Value{
+				"one": cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"value": cty.StringVal("val1"),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingMap,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String},
+						"value": {Type: cty.String},
+					},
+				},
+			},
+			// BUG: should be cty.MapVal with the input values preserved,
+			// but current code returns empty.
+			expected: cty.MapValEmpty(cty.Object(map[string]cty.Type{
+				"name":  cty.String,
+				"value": cty.String,
+			})),
+		},
+
+		// =====================================================================
+		// Deep nesting: NestingSingle containing NestingList
+		// =====================================================================
+
+		"nesting_single_containing_nesting_list": {
+			// A single object that has a child attribute which is a NestingList.
+			// The fillObject function recurses into fillAttribute for children,
+			// which then calls fillIterable for the NestingList child.
+			in: cty.ObjectVal(map[string]cty.Value{
+				"id": cty.StringVal("myid"),
+				"items": cty.TupleVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"name": cty.StringVal("item1"),
+					}),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingSingle,
+					Attributes: map[string]*configschema.Attribute{
+						"id": {Type: cty.String},
+						"items": {
+							NestedType: &configschema.Object{
+								Nesting: configschema.NestingList,
+								Attributes: map[string]*configschema.Attribute{
+									"name":  {Type: cty.String},
+									"value": {Type: cty.String},
+								},
+							},
+						},
+					},
+				},
+			},
+			expected: cty.ObjectVal(map[string]cty.Value{
+				"id": cty.StringVal("myid"),
+				"items": cty.ListVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"name":  cty.StringVal("item1"),
+						"value": cty.StringVal("ssnk9qhr"),
+					}),
+				}),
+			}),
+		},
+
+		// =====================================================================
+		// Deep nesting: NestingList containing NestingSingle
+		// =====================================================================
+
+		"nesting_list_containing_nesting_single": {
+			// List elements contain a NestingSingle child attribute.
+			// The fillIterable path goes through fillType which handles
+			// the list→object recursion, but nested NestedType attributes
+			// within the objects are handled purely at the cty.Type level
+			// since fillType doesn't know about configschema.Attribute.
+			// This works because ConfigType() produces the full type tree.
+			in: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name": cty.StringVal("item1"),
+					"detail": cty.ObjectVal(map[string]cty.Value{
+						"key": cty.StringVal("mykey"),
+					}),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"name": {Type: cty.String},
+						"detail": {
+							NestedType: &configschema.Object{
+								Nesting: configschema.NestingSingle,
+								Attributes: map[string]*configschema.Attribute{
+									"key":   {Type: cty.String},
+									"value": {Type: cty.String},
+								},
+							},
+						},
+					},
+				},
+			},
+			expected: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name": cty.StringVal("item1"),
+					"detail": cty.ObjectVal(map[string]cty.Value{
+						"key":   cty.StringVal("mykey"),
+						"value": cty.StringVal("ssnk9qhr"),
+					}),
+				}),
+			}),
+		},
+
+		// =====================================================================
+		// Deep nesting: NestingList containing NestingList (3 levels)
+		// =====================================================================
+
+		"nesting_list_containing_nesting_list": {
+			// Three levels: outer list → objects with inner list → objects.
+			// Tests that the fillIterable→fillType recursion handles nested
+			// list-of-object types correctly through pure cty.Type handling.
+			in: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"group": cty.StringVal("g1"),
+					"members": cty.TupleVal([]cty.Value{
+						cty.ObjectVal(map[string]cty.Value{
+							"name": cty.StringVal("alice"),
+						}),
+					}),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"group": {Type: cty.String},
+						"members": {
+							NestedType: &configschema.Object{
+								Nesting: configschema.NestingList,
+								Attributes: map[string]*configschema.Attribute{
+									"name": {Type: cty.String},
+									"role": {Type: cty.String},
+								},
+							},
+						},
+					},
+				},
+			},
+			expected: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"group": cty.StringVal("g1"),
+					"members": cty.ListVal([]cty.Value{
+						cty.ObjectVal(map[string]cty.Value{
+							"name": cty.StringVal("alice"),
+							"role": cty.StringVal("ssnk9qhr"),
+						}),
+					}),
+				}),
+			}),
+		},
+
+		// =====================================================================
+		// Deep nesting: NestingSingle containing NestingSingle (2 levels)
+		// =====================================================================
+
+		"nesting_single_containing_nesting_single": {
+			in: cty.ObjectVal(map[string]cty.Value{
+				"outer_name": cty.StringVal("outer"),
+				"inner": cty.ObjectVal(map[string]cty.Value{
+					"inner_name": cty.StringVal("inner"),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingSingle,
+					Attributes: map[string]*configschema.Attribute{
+						"outer_name": {Type: cty.String},
+						"inner": {
+							NestedType: &configschema.Object{
+								Nesting: configschema.NestingSingle,
+								Attributes: map[string]*configschema.Attribute{
+									"inner_name":  {Type: cty.String},
+									"inner_value": {Type: cty.String},
+								},
+							},
+						},
+					},
+				},
+			},
+			expected: cty.ObjectVal(map[string]cty.Value{
+				"outer_name": cty.StringVal("outer"),
+				"inner": cty.ObjectVal(map[string]cty.Value{
+					"inner_name":  cty.StringVal("inner"),
+					"inner_value": cty.StringVal("ssnk9qhr"),
+				}),
+			}),
+		},
+
+		// =====================================================================
+		// NestingSingle with mixed attribute types inside
+		// =====================================================================
+
+		"nesting_single_with_mixed_types": {
+			// Object with string, number, bool, and list attributes.
+			in: cty.ObjectVal(map[string]cty.Value{
+				"name":   cty.StringVal("test"),
+				"active": cty.True,
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingSingle,
+					Attributes: map[string]*configschema.Attribute{
+						"active": {Type: cty.Bool},
+						"count":  {Type: cty.Number},
+						"name":   {Type: cty.String},
+					},
+				},
+			},
+			// Sorted: active (provided), count (generated=0), name (provided)
+			expected: cty.ObjectVal(map[string]cty.Value{
+				"active": cty.True,
+				"count":  cty.Zero,
+				"name":   cty.StringVal("test"),
+			}),
+		},
+
+		// =====================================================================
+		// NestingList with elements that have no missing attributes
+		// =====================================================================
+
+		"nesting_list_no_missing_attrs_multiple_elements": {
+			in: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"value": cty.StringVal("one"),
+				}),
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("second"),
+					"value": cty.StringVal("two"),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String},
+						"value": {Type: cty.String},
+					},
+				},
+			},
+			expected: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("first"),
+					"value": cty.StringVal("one"),
+				}),
+				cty.ObjectVal(map[string]cty.Value{
+					"name":  cty.StringVal("second"),
+					"value": cty.StringVal("two"),
+				}),
+			}),
+		},
+
+		// =====================================================================
+		// NestingList with single-attr objects (minimal schema)
+		// =====================================================================
+
+		"nesting_list_single_attr_schema": {
+			in: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"id": {Type: cty.String},
+					},
+				},
+			},
+			expected: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"id": cty.StringVal("ssnk9qhr"),
+				}),
+			}),
+		},
+
+		// =====================================================================
+		// NestingSingle with a child that has a plain list attribute (Type not NestedType)
+		// =====================================================================
+
+		"nesting_single_with_plain_list_child": {
+			in: cty.ObjectVal(map[string]cty.Value{
+				"name": cty.StringVal("test"),
+				"tags": cty.ListVal([]cty.Value{
+					cty.StringVal("tag1"),
+					cty.StringVal("tag2"),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingSingle,
+					Attributes: map[string]*configschema.Attribute{
+						"name": {Type: cty.String},
+						"tags": {Type: cty.List(cty.String)},
+					},
+				},
+			},
+			expected: cty.ObjectVal(map[string]cty.Value{
+				"name": cty.StringVal("test"),
+				"tags": cty.ListVal([]cty.Value{
+					cty.StringVal("tag1"),
+					cty.StringVal("tag2"),
+				}),
+			}),
+		},
+
+		// =====================================================================
+		// NestingList with bool and number attribute types
+		// =====================================================================
+
+		"nesting_list_with_bool_and_number_attrs": {
+			in: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"name": cty.StringVal("item"),
+				}),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"active": {Type: cty.Bool},
+						"count":  {Type: cty.Number},
+						"name":   {Type: cty.String},
+					},
+				},
+			},
+			// Sorted: active (generated=false), count (generated=0), name (provided)
+			expected: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"active": cty.False,
+					"count":  cty.Zero,
+					"name":   cty.StringVal("item"),
+				}),
+			}),
+		},
+	}
+
+	for name, tc := range tcs {
+		t.Run(name, func(t *testing.T) {
+			testRand = rand.New(rand.NewSource(0))
+			defer func() {
+				testRand = nil
+			}()
+
+			actual, err := FillAttribute(tc.in, tc.attribute)
+			if err != nil {
+				t.Fatalf("unexpected error: %s", err)
+			}
+
+			if !tc.expected.RawEquals(actual) {
+				t.Errorf("\nexpected: %s\nactual:   %s", tc.expected.GoString(), actual.GoString())
+			}
+		})
+	}
+}
+
+// TestFillAttribute_Errors tests error cases for FillAttribute.
+func TestFillAttribute_Errors(t *testing.T) {
+	tcs := map[string]struct {
+		in        cty.Value
+		attribute *configschema.Attribute
+		err       string
+	}{
+		"nesting_single_non_object_input": {
+			// NestingSingle requires an object input; a string should fail.
+			in: cty.StringVal("not an object"),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingSingle,
+					Attributes: map[string]*configschema.Attribute{
+						"name": {Type: cty.String},
+					},
+				},
+			},
+			err: "incompatible types; expected object type, found string",
+		},
+		"nesting_group_non_object_input": {
+			in: cty.NumberIntVal(42),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingGroup,
+					Attributes: map[string]*configschema.Attribute{
+						"name": {Type: cty.String},
+					},
+				},
+			},
+			err: "incompatible types; expected object type, found number",
+		},
+		"nesting_list_incompatible_input": {
+			// Passing a plain string where a list of objects is expected.
+			in: cty.StringVal("not a list"),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"name": {Type: cty.String},
+					},
+				},
+			},
+			err: "incompatible types; expected list of object, found string",
+		},
+		"nesting_list_element_type_mismatch": {
+			// Tuple elements are strings, but the schema expects objects.
+			in: cty.TupleVal([]cty.Value{
+				cty.StringVal("not an object"),
+			}),
+			attribute: &configschema.Attribute{
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"name": {Type: cty.String},
+					},
+				},
+			},
+			err: "incompatible types; expected object, found string",
+		},
+		"plain_attribute_type_mismatch": {
+			// Plain attribute with completely incompatible type.
+			in: cty.ListValEmpty(cty.String),
+			attribute: &configschema.Attribute{
+				Type: cty.Bool,
+			},
+			err: "bool required, but have list of string",
+		},
+	}
+
+	for name, tc := range tcs {
+		t.Run(name, func(t *testing.T) {
+			actual, err := FillAttribute(tc.in, tc.attribute)
+			if err == nil {
+				t.Fatalf("expected error but got success with value: %s", actual.GoString())
+			}
+			if out := err.Error(); out != tc.err {
+				t.Errorf("\nexpected error: %s\nactual error:   %s", tc.err, out)
+			}
+			if actual != cty.NilVal {
+				t.Errorf("expected cty.NilVal on error but got: %s", actual.GoString())
+			}
+		})
+	}
+}
+
 func TestFillType(t *testing.T) {
 	tcs := map[string]struct {
 		in  cty.Value
diff --git a/internal/moduletest/mocking/values_test.go b/internal/moduletest/mocking/values_test.go
index 7ec12162b72d..9e4e624615d9 100644
--- a/internal/moduletest/mocking/values_test.go
+++ b/internal/moduletest/mocking/values_test.go
@@ -565,6 +565,9 @@ func TestComputedValuesForDataSource(t *testing.T) {
 					"value": cty.String,
 				})),
 			}),
+			expectedFailures: []string{
+				`Terraform could not compute a value for the target type list of object with the mocked data defined at :0,0-0 with the attribute ".nested": incompatible types; expected list of object, found object.`,
+			},
 		},
 		"nested_set_attribute": {
 			target: cty.ObjectVal(map[string]cty.Value{

From 320ec2c6895b09a1ab2f4c991b6fa82cbd244b42 Mon Sep 17 00:00:00 2001
From: Samsondeen Dare <samsondeen.dare@hashicorp.com>
Date: Mon, 23 Feb 2026 12:03:59 +0100
Subject: [PATCH 03/11] more tests

---
 internal/command/testing/test_provider.go  |   1 +
 internal/moduletest/mocking/fill_test.go   | 859 ---------------------
 internal/moduletest/mocking/values_test.go | 108 +++
 3 files changed, 109 insertions(+), 859 deletions(-)

diff --git a/internal/command/testing/test_provider.go b/internal/command/testing/test_provider.go
index e40bda8ca231..2f921975b3db 100644
--- a/internal/command/testing/test_provider.go
+++ b/internal/command/testing/test_provider.go
@@ -66,6 +66,7 @@ var (
 					},
 				},
 			},
+
 			"test_complex_data_source": {
 				Body: &configschema.Block{
 					Attributes: map[string]*configschema.Attribute{
diff --git a/internal/moduletest/mocking/fill_test.go b/internal/moduletest/mocking/fill_test.go
index 6701eb6e66d4..008c0fbe804c 100644
--- a/internal/moduletest/mocking/fill_test.go
+++ b/internal/moduletest/mocking/fill_test.go
@@ -8,867 +8,8 @@ import (
 	"testing"
 
 	"github.com/zclconf/go-cty/cty"
-
-	"github.com/hashicorp/terraform/internal/configs/configschema"
 )
 
-// TestFillAttribute tests the FillAttribute function which handles
-// configschema.Attribute values including NestedType attributes with various
-// nesting modes (NestingSingle, NestingGroup, NestingList, NestingSet, NestingMap).
-func TestFillAttribute(t *testing.T) {
-	tcs := map[string]struct {
-		in        cty.Value
-		attribute *configschema.Attribute
-		expected  cty.Value
-	}{
-		// =====================================================================
-		// Plain attributes (no NestedType) - falls through to fillType
-		// =====================================================================
-
-		"plain_string_attribute": {
-			in: cty.StringVal("hello"),
-			attribute: &configschema.Attribute{
-				Type: cty.String,
-			},
-			expected: cty.StringVal("hello"),
-		},
-
-		"plain_number_to_string_conversion": {
-			in: cty.NumberIntVal(42),
-			attribute: &configschema.Attribute{
-				Type: cty.String,
-			},
-			expected: cty.StringVal("42"),
-		},
-
-		"plain_list_of_objects_attribute": {
-			// When the attribute uses Type (not NestedType), it goes through
-			// fillType directly. This is the "plain" list-of-objects case
-			// that already worked before the fix.
-			in: cty.TupleVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name": cty.StringVal("first"),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				Type: cty.List(cty.Object(map[string]cty.Type{
-					"name":  cty.String,
-					"value": cty.String,
-				})),
-			},
-			expected: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"value": cty.StringVal("ssnk9qhr"),
-				}),
-			}),
-		},
-
-		// =====================================================================
-		// NestingSingle
-		// =====================================================================
-
-		"nesting_single_partial_attrs": {
-			in: cty.ObjectVal(map[string]cty.Value{
-				"name": cty.StringVal("hello"),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingSingle,
-					Attributes: map[string]*configschema.Attribute{
-						"name":  {Type: cty.String},
-						"value": {Type: cty.String},
-					},
-				},
-			},
-			expected: cty.ObjectVal(map[string]cty.Value{
-				"name":  cty.StringVal("hello"),
-				"value": cty.StringVal("ssnk9qhr"),
-			}),
-		},
-
-		"nesting_single_all_attrs_present": {
-			in: cty.ObjectVal(map[string]cty.Value{
-				"name":  cty.StringVal("hello"),
-				"value": cty.StringVal("world"),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingSingle,
-					Attributes: map[string]*configschema.Attribute{
-						"name":  {Type: cty.String},
-						"value": {Type: cty.String},
-					},
-				},
-			},
-			expected: cty.ObjectVal(map[string]cty.Value{
-				"name":  cty.StringVal("hello"),
-				"value": cty.StringVal("world"),
-			}),
-		},
-
-		"nesting_single_empty_schema": {
-			in: cty.EmptyObjectVal,
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting:    configschema.NestingSingle,
-					Attributes: map[string]*configschema.Attribute{},
-				},
-			},
-			expected: cty.EmptyObjectVal,
-		},
-
-		"nesting_single_extra_attrs_in_input_dropped": {
-			// Input has attributes not in the schema; they should be dropped.
-			in: cty.ObjectVal(map[string]cty.Value{
-				"name":  cty.StringVal("hello"),
-				"extra": cty.StringVal("should be dropped"),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingSingle,
-					Attributes: map[string]*configschema.Attribute{
-						"name": {Type: cty.String},
-					},
-				},
-			},
-			expected: cty.ObjectVal(map[string]cty.Value{
-				"name": cty.StringVal("hello"),
-			}),
-		},
-
-		// =====================================================================
-		// NestingGroup (behaves same as NestingSingle for fill purposes)
-		// =====================================================================
-
-		"nesting_group_partial_attrs": {
-			in: cty.ObjectVal(map[string]cty.Value{
-				"name": cty.StringVal("hello"),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingGroup,
-					Attributes: map[string]*configschema.Attribute{
-						"name":  {Type: cty.String},
-						"value": {Type: cty.String},
-					},
-				},
-			},
-			expected: cty.ObjectVal(map[string]cty.Value{
-				"name":  cty.StringVal("hello"),
-				"value": cty.StringVal("ssnk9qhr"),
-			}),
-		},
-
-		// =====================================================================
-		// NestingList - the main bug fix (GitHub issue #37939)
-		// =====================================================================
-
-		"nesting_list_tuple_input_partial_attrs": {
-			// This is the core bug scenario: HCL produces a tuple when parsing
-			// list literals in override_data values. The old code would fail
-			// or return empty for this input.
-			in: cty.TupleVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name": cty.StringVal("first"),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"name":  {Type: cty.String},
-						"value": {Type: cty.String},
-					},
-				},
-			},
-			expected: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"value": cty.StringVal("ssnk9qhr"),
-				}),
-			}),
-		},
-
-		"nesting_list_list_input_partial_attrs": {
-			// Input is already a proper list (not a tuple).
-			in: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name": cty.StringVal("first"),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"name":  {Type: cty.String},
-						"value": {Type: cty.String},
-					},
-				},
-			},
-			expected: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"value": cty.StringVal("ssnk9qhr"),
-				}),
-			}),
-		},
-
-		"nesting_list_multiple_elements": {
-			in: cty.TupleVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name": cty.StringVal("first"),
-				}),
-				cty.ObjectVal(map[string]cty.Value{
-					"name": cty.StringVal("second"),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"name":  {Type: cty.String},
-						"value": {Type: cty.String},
-					},
-				},
-			},
-			expected: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"value": cty.StringVal("ssnk9qhr"),
-				}),
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("second"),
-					"value": cty.StringVal("amyllmyg"),
-				}),
-			}),
-		},
-
-		"nesting_list_all_attrs_present": {
-			in: cty.TupleVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"value": cty.StringVal("one"),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"name":  {Type: cty.String},
-						"value": {Type: cty.String},
-					},
-				},
-			},
-			expected: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"value": cty.StringVal("one"),
-				}),
-			}),
-		},
-
-		"nesting_list_extra_attrs_in_elements_dropped": {
-			// Input objects have attributes not defined in the schema.
-			in: cty.TupleVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"value": cty.StringVal("one"),
-					"extra": cty.StringVal("should be dropped"),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"name":  {Type: cty.String},
-						"value": {Type: cty.String},
-					},
-				},
-			},
-			expected: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"value": cty.StringVal("one"),
-				}),
-			}),
-		},
-
-		"nesting_list_set_input": {
-			// A set can also be converted to a list.
-			in: cty.SetVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"value": cty.StringVal("one"),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"name":  {Type: cty.String},
-						"value": {Type: cty.String},
-					},
-				},
-			},
-			expected: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"value": cty.StringVal("one"),
-				}),
-			}),
-		},
-
-		"nesting_list_element_type_conversion": {
-			// Elements have attributes that need type conversion (number→string).
-			in: cty.TupleVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"count": cty.NumberIntVal(42),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"name":  {Type: cty.String},
-						"count": {Type: cty.String},
-					},
-				},
-			},
-			expected: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"count": cty.StringVal("42"),
-				}),
-			}),
-		},
-
-		"nesting_list_three_attrs_partial": {
-			// Object schema with 3 attributes, only 1 provided.
-			// Attributes are filled in alphabetical order: "alpha" (provided),
-			// "beta" (generated=ssnk9qhr), "gamma" (generated=amyllmyg).
-			in: cty.TupleVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"alpha": cty.StringVal("provided"),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"alpha": {Type: cty.String},
-						"beta":  {Type: cty.String},
-						"gamma": {Type: cty.String},
-					},
-				},
-			},
-			expected: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"alpha": cty.StringVal("provided"),
-					"beta":  cty.StringVal("ssnk9qhr"),
-					"gamma": cty.StringVal("amyllmyg"),
-				}),
-			}),
-		},
-
-		// =====================================================================
-		// NestingSet - BUG: current implementation ignores input and always
-		// returns an empty set. These tests document the CURRENT behavior.
-		// If NestingSet is fixed to use fillIterable (like NestingList),
-		// these tests should be updated to expect filled values.
-		// =====================================================================
-
-		"nesting_set_returns_empty_ignoring_input": {
-			// BUG: The current implementation returns an empty set regardless
-			// of the input. Override values for NestingSet attributes are
-			// silently dropped, just like the original NestingList bug.
-			in: cty.SetVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"value": cty.StringVal("one"),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingSet,
-					Attributes: map[string]*configschema.Attribute{
-						"name":  {Type: cty.String},
-						"value": {Type: cty.String},
-					},
-				},
-			},
-			// BUG: should be cty.SetVal with the input values preserved,
-			// but current code returns empty.
-			expected: cty.SetValEmpty(cty.Object(map[string]cty.Type{
-				"name":  cty.String,
-				"value": cty.String,
-			})),
-		},
-
-		// =====================================================================
-		// NestingMap - BUG: current implementation ignores input and always
-		// returns an empty map. These tests document the CURRENT behavior.
-		// If NestingMap is fixed to use fillIterable (like NestingList),
-		// these tests should be updated to expect filled values.
-		// =====================================================================
-
-		"nesting_map_returns_empty_ignoring_input": {
-			// BUG: The current implementation returns an empty map regardless
-			// of the input. Override values for NestingMap attributes are
-			// silently dropped, just like the original NestingList bug.
-			in: cty.MapVal(map[string]cty.Value{
-				"one": cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"value": cty.StringVal("val1"),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingMap,
-					Attributes: map[string]*configschema.Attribute{
-						"name":  {Type: cty.String},
-						"value": {Type: cty.String},
-					},
-				},
-			},
-			// BUG: should be cty.MapVal with the input values preserved,
-			// but current code returns empty.
-			expected: cty.MapValEmpty(cty.Object(map[string]cty.Type{
-				"name":  cty.String,
-				"value": cty.String,
-			})),
-		},
-
-		// =====================================================================
-		// Deep nesting: NestingSingle containing NestingList
-		// =====================================================================
-
-		"nesting_single_containing_nesting_list": {
-			// A single object that has a child attribute which is a NestingList.
-			// The fillObject function recurses into fillAttribute for children,
-			// which then calls fillIterable for the NestingList child.
-			in: cty.ObjectVal(map[string]cty.Value{
-				"id": cty.StringVal("myid"),
-				"items": cty.TupleVal([]cty.Value{
-					cty.ObjectVal(map[string]cty.Value{
-						"name": cty.StringVal("item1"),
-					}),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingSingle,
-					Attributes: map[string]*configschema.Attribute{
-						"id": {Type: cty.String},
-						"items": {
-							NestedType: &configschema.Object{
-								Nesting: configschema.NestingList,
-								Attributes: map[string]*configschema.Attribute{
-									"name":  {Type: cty.String},
-									"value": {Type: cty.String},
-								},
-							},
-						},
-					},
-				},
-			},
-			expected: cty.ObjectVal(map[string]cty.Value{
-				"id": cty.StringVal("myid"),
-				"items": cty.ListVal([]cty.Value{
-					cty.ObjectVal(map[string]cty.Value{
-						"name":  cty.StringVal("item1"),
-						"value": cty.StringVal("ssnk9qhr"),
-					}),
-				}),
-			}),
-		},
-
-		// =====================================================================
-		// Deep nesting: NestingList containing NestingSingle
-		// =====================================================================
-
-		"nesting_list_containing_nesting_single": {
-			// List elements contain a NestingSingle child attribute.
-			// The fillIterable path goes through fillType which handles
-			// the list→object recursion, but nested NestedType attributes
-			// within the objects are handled purely at the cty.Type level
-			// since fillType doesn't know about configschema.Attribute.
-			// This works because ConfigType() produces the full type tree.
-			in: cty.TupleVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name": cty.StringVal("item1"),
-					"detail": cty.ObjectVal(map[string]cty.Value{
-						"key": cty.StringVal("mykey"),
-					}),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"name": {Type: cty.String},
-						"detail": {
-							NestedType: &configschema.Object{
-								Nesting: configschema.NestingSingle,
-								Attributes: map[string]*configschema.Attribute{
-									"key":   {Type: cty.String},
-									"value": {Type: cty.String},
-								},
-							},
-						},
-					},
-				},
-			},
-			expected: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name": cty.StringVal("item1"),
-					"detail": cty.ObjectVal(map[string]cty.Value{
-						"key":   cty.StringVal("mykey"),
-						"value": cty.StringVal("ssnk9qhr"),
-					}),
-				}),
-			}),
-		},
-
-		// =====================================================================
-		// Deep nesting: NestingList containing NestingList (3 levels)
-		// =====================================================================
-
-		"nesting_list_containing_nesting_list": {
-			// Three levels: outer list → objects with inner list → objects.
-			// Tests that the fillIterable→fillType recursion handles nested
-			// list-of-object types correctly through pure cty.Type handling.
-			in: cty.TupleVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"group": cty.StringVal("g1"),
-					"members": cty.TupleVal([]cty.Value{
-						cty.ObjectVal(map[string]cty.Value{
-							"name": cty.StringVal("alice"),
-						}),
-					}),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"group": {Type: cty.String},
-						"members": {
-							NestedType: &configschema.Object{
-								Nesting: configschema.NestingList,
-								Attributes: map[string]*configschema.Attribute{
-									"name": {Type: cty.String},
-									"role": {Type: cty.String},
-								},
-							},
-						},
-					},
-				},
-			},
-			expected: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"group": cty.StringVal("g1"),
-					"members": cty.ListVal([]cty.Value{
-						cty.ObjectVal(map[string]cty.Value{
-							"name": cty.StringVal("alice"),
-							"role": cty.StringVal("ssnk9qhr"),
-						}),
-					}),
-				}),
-			}),
-		},
-
-		// =====================================================================
-		// Deep nesting: NestingSingle containing NestingSingle (2 levels)
-		// =====================================================================
-
-		"nesting_single_containing_nesting_single": {
-			in: cty.ObjectVal(map[string]cty.Value{
-				"outer_name": cty.StringVal("outer"),
-				"inner": cty.ObjectVal(map[string]cty.Value{
-					"inner_name": cty.StringVal("inner"),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingSingle,
-					Attributes: map[string]*configschema.Attribute{
-						"outer_name": {Type: cty.String},
-						"inner": {
-							NestedType: &configschema.Object{
-								Nesting: configschema.NestingSingle,
-								Attributes: map[string]*configschema.Attribute{
-									"inner_name":  {Type: cty.String},
-									"inner_value": {Type: cty.String},
-								},
-							},
-						},
-					},
-				},
-			},
-			expected: cty.ObjectVal(map[string]cty.Value{
-				"outer_name": cty.StringVal("outer"),
-				"inner": cty.ObjectVal(map[string]cty.Value{
-					"inner_name":  cty.StringVal("inner"),
-					"inner_value": cty.StringVal("ssnk9qhr"),
-				}),
-			}),
-		},
-
-		// =====================================================================
-		// NestingSingle with mixed attribute types inside
-		// =====================================================================
-
-		"nesting_single_with_mixed_types": {
-			// Object with string, number, bool, and list attributes.
-			in: cty.ObjectVal(map[string]cty.Value{
-				"name":   cty.StringVal("test"),
-				"active": cty.True,
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingSingle,
-					Attributes: map[string]*configschema.Attribute{
-						"active": {Type: cty.Bool},
-						"count":  {Type: cty.Number},
-						"name":   {Type: cty.String},
-					},
-				},
-			},
-			// Sorted: active (provided), count (generated=0), name (provided)
-			expected: cty.ObjectVal(map[string]cty.Value{
-				"active": cty.True,
-				"count":  cty.Zero,
-				"name":   cty.StringVal("test"),
-			}),
-		},
-
-		// =====================================================================
-		// NestingList with elements that have no missing attributes
-		// =====================================================================
-
-		"nesting_list_no_missing_attrs_multiple_elements": {
-			in: cty.TupleVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"value": cty.StringVal("one"),
-				}),
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("second"),
-					"value": cty.StringVal("two"),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"name":  {Type: cty.String},
-						"value": {Type: cty.String},
-					},
-				},
-			},
-			expected: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("first"),
-					"value": cty.StringVal("one"),
-				}),
-				cty.ObjectVal(map[string]cty.Value{
-					"name":  cty.StringVal("second"),
-					"value": cty.StringVal("two"),
-				}),
-			}),
-		},
-
-		// =====================================================================
-		// NestingList with single-attr objects (minimal schema)
-		// =====================================================================
-
-		"nesting_list_single_attr_schema": {
-			in: cty.TupleVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"id": {Type: cty.String},
-					},
-				},
-			},
-			expected: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"id": cty.StringVal("ssnk9qhr"),
-				}),
-			}),
-		},
-
-		// =====================================================================
-		// NestingSingle with a child that has a plain list attribute (Type not NestedType)
-		// =====================================================================
-
-		"nesting_single_with_plain_list_child": {
-			in: cty.ObjectVal(map[string]cty.Value{
-				"name": cty.StringVal("test"),
-				"tags": cty.ListVal([]cty.Value{
-					cty.StringVal("tag1"),
-					cty.StringVal("tag2"),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingSingle,
-					Attributes: map[string]*configschema.Attribute{
-						"name": {Type: cty.String},
-						"tags": {Type: cty.List(cty.String)},
-					},
-				},
-			},
-			expected: cty.ObjectVal(map[string]cty.Value{
-				"name": cty.StringVal("test"),
-				"tags": cty.ListVal([]cty.Value{
-					cty.StringVal("tag1"),
-					cty.StringVal("tag2"),
-				}),
-			}),
-		},
-
-		// =====================================================================
-		// NestingList with bool and number attribute types
-		// =====================================================================
-
-		"nesting_list_with_bool_and_number_attrs": {
-			in: cty.TupleVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"name": cty.StringVal("item"),
-				}),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"active": {Type: cty.Bool},
-						"count":  {Type: cty.Number},
-						"name":   {Type: cty.String},
-					},
-				},
-			},
-			// Sorted: active (generated=false), count (generated=0), name (provided)
-			expected: cty.ListVal([]cty.Value{
-				cty.ObjectVal(map[string]cty.Value{
-					"active": cty.False,
-					"count":  cty.Zero,
-					"name":   cty.StringVal("item"),
-				}),
-			}),
-		},
-	}
-
-	for name, tc := range tcs {
-		t.Run(name, func(t *testing.T) {
-			testRand = rand.New(rand.NewSource(0))
-			defer func() {
-				testRand = nil
-			}()
-
-			actual, err := FillAttribute(tc.in, tc.attribute)
-			if err != nil {
-				t.Fatalf("unexpected error: %s", err)
-			}
-
-			if !tc.expected.RawEquals(actual) {
-				t.Errorf("\nexpected: %s\nactual:   %s", tc.expected.GoString(), actual.GoString())
-			}
-		})
-	}
-}
-
-// TestFillAttribute_Errors tests error cases for FillAttribute.
-func TestFillAttribute_Errors(t *testing.T) {
-	tcs := map[string]struct {
-		in        cty.Value
-		attribute *configschema.Attribute
-		err       string
-	}{
-		"nesting_single_non_object_input": {
-			// NestingSingle requires an object input; a string should fail.
-			in: cty.StringVal("not an object"),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingSingle,
-					Attributes: map[string]*configschema.Attribute{
-						"name": {Type: cty.String},
-					},
-				},
-			},
-			err: "incompatible types; expected object type, found string",
-		},
-		"nesting_group_non_object_input": {
-			in: cty.NumberIntVal(42),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingGroup,
-					Attributes: map[string]*configschema.Attribute{
-						"name": {Type: cty.String},
-					},
-				},
-			},
-			err: "incompatible types; expected object type, found number",
-		},
-		"nesting_list_incompatible_input": {
-			// Passing a plain string where a list of objects is expected.
-			in: cty.StringVal("not a list"),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"name": {Type: cty.String},
-					},
-				},
-			},
-			err: "incompatible types; expected list of object, found string",
-		},
-		"nesting_list_element_type_mismatch": {
-			// Tuple elements are strings, but the schema expects objects.
-			in: cty.TupleVal([]cty.Value{
-				cty.StringVal("not an object"),
-			}),
-			attribute: &configschema.Attribute{
-				NestedType: &configschema.Object{
-					Nesting: configschema.NestingList,
-					Attributes: map[string]*configschema.Attribute{
-						"name": {Type: cty.String},
-					},
-				},
-			},
-			err: "incompatible types; expected object, found string",
-		},
-		"plain_attribute_type_mismatch": {
-			// Plain attribute with completely incompatible type.
-			in: cty.ListValEmpty(cty.String),
-			attribute: &configschema.Attribute{
-				Type: cty.Bool,
-			},
-			err: "bool required, but have list of string",
-		},
-	}
-
-	for name, tc := range tcs {
-		t.Run(name, func(t *testing.T) {
-			actual, err := FillAttribute(tc.in, tc.attribute)
-			if err == nil {
-				t.Fatalf("expected error but got success with value: %s", actual.GoString())
-			}
-			if out := err.Error(); out != tc.err {
-				t.Errorf("\nexpected error: %s\nactual error:   %s", tc.err, out)
-			}
-			if actual != cty.NilVal {
-				t.Errorf("expected cty.NilVal on error but got: %s", actual.GoString())
-			}
-		})
-	}
-}
-
 func TestFillType(t *testing.T) {
 	tcs := map[string]struct {
 		in  cty.Value
diff --git a/internal/moduletest/mocking/values_test.go b/internal/moduletest/mocking/values_test.go
index 9e4e624615d9..1757be11cc51 100644
--- a/internal/moduletest/mocking/values_test.go
+++ b/internal/moduletest/mocking/values_test.go
@@ -537,6 +537,40 @@ func TestComputedValuesForDataSource(t *testing.T) {
 			}),
 		},
 		"nested_list_attribute_computed": {
+			target: cty.ObjectVal(map[string]cty.Value{
+				"nested": cty.NullVal(cty.List(cty.Object(map[string]cty.Type{
+					"id":    cty.String,
+					"value": cty.String,
+				}))),
+			}),
+			with: cty.ObjectVal(map[string]cty.Value{
+				"nested": cty.ListVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"id": cty.StringVal("myvalue"),
+					}),
+				}),
+			}),
+			schema: &configschema.Block{
+				Attributes: map[string]*configschema.Attribute{
+					"nested": {
+						NestedType: &configschema.Object{
+							Attributes: computedAttributes,
+							Nesting:    configschema.NestingList,
+						},
+						Computed: true,
+					},
+				},
+			},
+			expected: cty.ObjectVal(map[string]cty.Value{
+				"nested": cty.ListVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"id":    cty.StringVal("myvalue"),
+						"value": cty.StringVal("ssnk9qhr"),
+					}),
+				}),
+			}),
+		},
+		"nested_list_attribute_computed_err": {
 			target: cty.ObjectVal(map[string]cty.Value{
 				"nested": cty.NullVal(cty.List(cty.Object(map[string]cty.Type{
 					"id":    cty.String,
@@ -637,6 +671,40 @@ func TestComputedValuesForDataSource(t *testing.T) {
 			}),
 		},
 		"nested_set_attribute_computed": {
+			target: cty.ObjectVal(map[string]cty.Value{
+				"nested": cty.NullVal(cty.Set(cty.Object(map[string]cty.Type{
+					"id":    cty.String,
+					"value": cty.String,
+				}))),
+			}),
+			with: cty.ObjectVal(map[string]cty.Value{
+				"nested": cty.SetVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"id": cty.StringVal("myvalue"),
+					}),
+				}),
+			}),
+			schema: &configschema.Block{
+				Attributes: map[string]*configschema.Attribute{
+					"nested": {
+						NestedType: &configschema.Object{
+							Attributes: computedAttributes,
+							Nesting:    configschema.NestingSet,
+						},
+						Computed: true,
+					},
+				},
+			},
+			expected: cty.ObjectVal(map[string]cty.Value{
+				"nested": cty.SetVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"id":    cty.StringVal("myvalue"),
+						"value": cty.StringVal("ssnk9qhr"),
+					}),
+				}),
+			}),
+		},
+		"nested_set_attribute_computed_err": {
 			target: cty.ObjectVal(map[string]cty.Value{
 				"nested": cty.NullVal(cty.Set(cty.Object(map[string]cty.Type{
 					"id":    cty.String,
@@ -665,6 +733,9 @@ func TestComputedValuesForDataSource(t *testing.T) {
 					"value": cty.String,
 				})),
 			}),
+			expectedFailures: []string{
+				`Terraform could not compute a value for the target type set of object with the mocked data defined at :0,0-0 with the attribute ".nested": incompatible types; expected set of object, found object.`,
+			},
 		},
 		"nested_map_attribute": {
 			target: cty.ObjectVal(map[string]cty.Value{
@@ -734,6 +805,40 @@ func TestComputedValuesForDataSource(t *testing.T) {
 			}),
 		},
 		"nested_map_attribute_computed": {
+			target: cty.ObjectVal(map[string]cty.Value{
+				"nested": cty.NullVal(cty.Map(cty.Object(map[string]cty.Type{
+					"id":    cty.String,
+					"value": cty.String,
+				}))),
+			}),
+			with: cty.ObjectVal(map[string]cty.Value{
+				"nested": cty.MapVal(map[string]cty.Value{
+					"key": cty.ObjectVal(map[string]cty.Value{
+						"id": cty.StringVal("myvalue"),
+					}),
+				}),
+			}),
+			schema: &configschema.Block{
+				Attributes: map[string]*configschema.Attribute{
+					"nested": {
+						NestedType: &configschema.Object{
+							Attributes: computedAttributes,
+							Nesting:    configschema.NestingMap,
+						},
+						Computed: true,
+					},
+				},
+			},
+			expected: cty.ObjectVal(map[string]cty.Value{
+				"nested": cty.MapVal(map[string]cty.Value{
+					"key": cty.ObjectVal(map[string]cty.Value{
+						"id":    cty.StringVal("myvalue"),
+						"value": cty.StringVal("ssnk9qhr"),
+					}),
+				}),
+			}),
+		},
+		"nested_map_attribute_computed_err": {
 			target: cty.ObjectVal(map[string]cty.Value{
 				"nested": cty.NullVal(cty.Map(cty.Object(map[string]cty.Type{
 					"id":    cty.String,
@@ -762,6 +867,9 @@ func TestComputedValuesForDataSource(t *testing.T) {
 					"value": cty.String,
 				})),
 			}),
+			expectedFailures: []string{
+				`Terraform could not compute a value for the target type map of object with the mocked data defined at :0,0-0 with the attribute ".nested[\"id\"]": incompatible types; expected object, found string.`,
+			},
 		},
 		"invalid_replacement_path": {
 			target: cty.ObjectVal(map[string]cty.Value{

From 7951e69dcdde4268e4c5489d2dc96c69121c8282 Mon Sep 17 00:00:00 2001
From: Samsondeen Dare <samsondeen.dare@hashicorp.com>
Date: Mon, 23 Feb 2026 12:43:25 +0100
Subject: [PATCH 04/11] remove fillIterable

---
 internal/moduletest/mocking/fill.go | 54 ++---------------------------
 1 file changed, 2 insertions(+), 52 deletions(-)

diff --git a/internal/moduletest/mocking/fill.go b/internal/moduletest/mocking/fill.go
index 676bf815893f..469e2735378b 100644
--- a/internal/moduletest/mocking/fill.go
+++ b/internal/moduletest/mocking/fill.go
@@ -22,61 +22,11 @@ func FillAttribute(in cty.Value, attribute *configschema.Attribute) (cty.Value,
 }
 
 func fillAttribute(in cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
+	ty := attribute.Type
 	if attribute.NestedType != nil {
-
-		switch attribute.NestedType.Nesting {
-		case configschema.NestingSingle, configschema.NestingGroup:
-			return fillObject(in, attribute, path)
-		case configschema.NestingSet:
-			return fillIterable(in, attribute, path)
-		case configschema.NestingList:
-			return fillIterable(in, attribute, path)
-		case configschema.NestingMap:
-			return fillIterable(in, attribute, path)
-		default:
-			panic(fmt.Errorf("unknown nesting mode: %d", attribute.NestedType.Nesting))
-		}
-	}
-
-	return fillType(in, attribute.Type, path)
-}
-
-func fillObject(in cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
-	// Then the in value must be an object.
-	if !in.Type().IsObjectType() {
-		return cty.NilVal, path.NewErrorf("incompatible types; expected object type, found %s", in.Type().FriendlyName())
-	}
-
-	var names []string
-	for name := range attribute.NestedType.Attributes {
-		names = append(names, name)
+		ty = attribute.NestedType.ConfigType()
 	}
-	if len(names) == 0 {
-		return cty.EmptyObjectVal, nil
-	}
-
-	// Make the order we iterate through the attributes deterministic. We
-	// are generating random strings in here so it's worth making the
-	// operation repeatable.
-	sort.Strings(names)
-
-	children := make(map[string]cty.Value)
-	for _, name := range names {
-		if in.Type().HasAttribute(name) {
-			child, err := fillAttribute(in.GetAttr(name), attribute.NestedType.Attributes[name], path.GetAttr(name))
-			if err != nil {
-				return cty.NilVal, err
-			}
-			children[name] = child
-			continue
-		}
-		children[name] = GenerateValueForAttribute(attribute.NestedType.Attributes[name])
-	}
-	return cty.ObjectVal(children), nil
-}
 
-func fillIterable(in cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
-	ty := attribute.NestedType.ConfigType()
 	return fillType(in, ty, path)
 }
 

From bfe600de3dee9b460b0394ac52b284ea7ecc0399 Mon Sep 17 00:00:00 2001
From: Samsondeen Dare <samsondeen.dare@hashicorp.com>
Date: Mon, 23 Feb 2026 13:07:11 +0100
Subject: [PATCH 05/11] add changelog

---
 .changes/v1.15/ENHANCEMENTS-20260223-130341.yaml | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changes/v1.15/ENHANCEMENTS-20260223-130341.yaml

diff --git a/.changes/v1.15/ENHANCEMENTS-20260223-130341.yaml b/.changes/v1.15/ENHANCEMENTS-20260223-130341.yaml
new file mode 100644
index 000000000000..1393968cfc27
--- /dev/null
+++ b/.changes/v1.15/ENHANCEMENTS-20260223-130341.yaml
@@ -0,0 +1,5 @@
+kind: ENHANCEMENTS
+body: "mocking: computed values are now correctly filled for nested list, set, and map attributes when using mock and override data in tests."
+time: 2026-02-23T13:03:41.913383+01:00
+custom:
+  Issue: "37939"

From 6089bd986b12164136c1efca12b9efe7812c5160 Mon Sep 17 00:00:00 2001
From: Samsondeen Dare <samsondeen.dare@hashicorp.com>
Date: Mon, 23 Feb 2026 13:37:54 +0100
Subject: [PATCH 06/11] merge tests

---
 internal/command/test_test.go                 | 129 ++++++------------
 .../mocking/tests/nested_mocked.tftest.hcl    |  29 ++++
 .../list.tftest.hcl}                          |   0
 .../main.tf                                   |   0
 .../map.tftest.hcl}                           |   0
 .../nested_list.tftest.hcl}                   |   0
 .../nested_map.tftest.hcl}                    |   0
 .../nested_set.tftest.hcl}                    |   0
 .../set.tftest.hcl}                           |   0
 .../test/override_data_list_attribute/main.tf |  15 --
 .../test/override_data_map_attribute/main.tf  |  15 --
 .../main.tf                                   |  15 --
 .../main.tf                                   |  15 --
 .../main.tf                                   |  15 --
 14 files changed, 72 insertions(+), 161 deletions(-)
 create mode 100644 internal/command/testdata/test/mocking/tests/nested_mocked.tftest.hcl
 rename internal/command/testdata/test/{override_data_list_attribute/main.tftest.hcl => override_data_attribute/list.tftest.hcl} (100%)
 rename internal/command/testdata/test/{override_data_set_attribute => override_data_attribute}/main.tf (100%)
 rename internal/command/testdata/test/{override_data_map_attribute/main.tftest.hcl => override_data_attribute/map.tftest.hcl} (100%)
 rename internal/command/testdata/test/{override_data_nested_list_attribute/main.tftest.hcl => override_data_attribute/nested_list.tftest.hcl} (100%)
 rename internal/command/testdata/test/{override_data_nested_map_attribute/main.tftest.hcl => override_data_attribute/nested_map.tftest.hcl} (100%)
 rename internal/command/testdata/test/{override_data_nested_set_attribute/main.tftest.hcl => override_data_attribute/nested_set.tftest.hcl} (100%)
 rename internal/command/testdata/test/{override_data_set_attribute/main.tftest.hcl => override_data_attribute/set.tftest.hcl} (100%)
 delete mode 100644 internal/command/testdata/test/override_data_list_attribute/main.tf
 delete mode 100644 internal/command/testdata/test/override_data_map_attribute/main.tf
 delete mode 100644 internal/command/testdata/test/override_data_nested_list_attribute/main.tf
 delete mode 100644 internal/command/testdata/test/override_data_nested_map_attribute/main.tf
 delete mode 100644 internal/command/testdata/test/override_data_nested_set_attribute/main.tf

diff --git a/internal/command/test_test.go b/internal/command/test_test.go
index a27a34dc96fc..e163db8380f1 100644
--- a/internal/command/test_test.go
+++ b/internal/command/test_test.go
@@ -292,7 +292,7 @@ func TestTest_Runs(t *testing.T) {
 			code:        0,
 		},
 		"mocking": {
-			expectedOut: []string{"10 passed, 0 failed."},
+			expectedOut: []string{"11 passed, 0 failed."},
 			code:        0,
 		},
 		"mocking-invalid": {
@@ -5675,101 +5675,58 @@ func TestTest_TeardownOrder(t *testing.T) {
 }
 
 func TestTest_OverrideData(t *testing.T) {
-	tcs := map[string]struct {
-		dir  string
-		code int
-		desc string
-	}{
-		"list_attribute": {
-			dir:  "override_data_list_attribute",
-			code: 0,
-			desc: "override_data with a computed cty.List(cty.Object) attribute",
-		},
-		"nested_list_attribute": {
-			dir:  "override_data_nested_list_attribute",
-			code: 0,
-			desc: "override_data with a computed NestedType NestingList attribute",
-		},
-		"set_attribute": {
-			dir:  "override_data_set_attribute",
-			code: 0,
-			desc: "override_data with a computed cty.Set(cty.Object) attribute",
-		},
-		"nested_set_attribute": {
-			dir:  "override_data_nested_set_attribute",
-			code: 0,
-			desc: "override_data with a computed NestedType NestingSet attribute",
-		},
-		"map_attribute": {
-			dir:  "override_data_map_attribute",
-			code: 0,
-			desc: "override_data with a computed cty.Map(cty.Object) attribute",
-		},
-		"nested_map_attribute": {
-			dir:  "override_data_nested_map_attribute",
-			code: 0,
-			desc: "override_data with a computed NestedType NestingMap attribute",
-		},
-	}
-
-	for name, tc := range tcs {
-		t.Run(name, func(t *testing.T) {
-			td := t.TempDir()
-			testCopyDir(t, testFixturePath(path.Join("test", tc.dir)), td)
-			t.Chdir(td)
+	td := t.TempDir()
+	testCopyDir(t, testFixturePath(path.Join("test", "override_data_attribute")), td)
+	t.Chdir(td)
 
-			provider := testing_command.NewProvider(nil)
-			providerSource, closeFn := newMockProviderSource(t, map[string][]string{
-				"test": {"1.0.0"},
-			})
-			defer closeFn()
+	provider := testing_command.NewProvider(nil)
+	providerSource, closeFn := newMockProviderSource(t, map[string][]string{
+		"test": {"1.0.0"},
+	})
+	defer closeFn()
 
-			streams, done := terminal.StreamsForTesting(t)
-			view := views.NewView(streams)
-			ui := new(cli.MockUi)
+	streams, done := terminal.StreamsForTesting(t)
+	view := views.NewView(streams)
+	ui := new(cli.MockUi)
 
-			meta := Meta{
-				testingOverrides: metaOverridesForProvider(provider.Provider),
-				Ui:               ui,
-				View:             view,
-				Streams:          streams,
-				ProviderSource:   providerSource,
-			}
+	meta := Meta{
+		testingOverrides: metaOverridesForProvider(provider.Provider),
+		Ui:               ui,
+		View:             view,
+		Streams:          streams,
+		ProviderSource:   providerSource,
+	}
 
-			init := &InitCommand{
-				Meta: meta,
-			}
+	init := &InitCommand{
+		Meta: meta,
+	}
 
-			if code := init.Run(nil); code != 0 {
-				output := done(t)
-				t.Fatalf("expected init status code 0 but got %d: %s", code, output.All())
-			}
+	if code := init.Run(nil); code != 0 {
+		output := done(t)
+		t.Fatalf("expected init status code 0 but got %d: %s", code, output.All())
+	}
 
-			// Reset the streams for the next command.
-			streams, done = terminal.StreamsForTesting(t)
-			meta.Streams = streams
-			meta.View = views.NewView(streams)
+	// Reset the streams for the next command.
+	streams, done = terminal.StreamsForTesting(t)
+	meta.Streams = streams
+	meta.View = views.NewView(streams)
 
-			c := &TestCommand{
-				Meta: meta,
-			}
+	c := &TestCommand{
+		Meta: meta,
+	}
 
-			code := c.Run([]string{"-no-color"})
-			output := done(t)
+	code := c.Run([]string{"-no-color"})
+	output := done(t)
 
-			if code != tc.code {
-				t.Errorf("expected status code %d but got %d:\n\n%s", tc.code, code, output.All())
-			}
+	if code != 0 {
+		t.Errorf("expected status code %d but got %d:\n\n%s", 0, code, output.All())
+	}
 
-			if tc.code == 0 {
-				if !strings.Contains(output.Stdout(), "1 passed, 0 failed.") {
-					t.Errorf("expected passing test output but got:\n\nstdout:\n%s\nstderr:\n%s", output.Stdout(), output.Stderr())
-				}
-				if output.Stderr() != "" {
-					t.Errorf("unexpected stderr output:\n%s", output.Stderr())
-				}
-			}
-		})
+	if !strings.Contains(output.Stdout(), "6 passed, 0 failed.") {
+		t.Errorf("expected passing test output but got:\n\nstdout:\n%s\nstderr:\n%s", output.Stdout(), output.Stderr())
+	}
+	if output.Stderr() != "" {
+		t.Errorf("unexpected stderr output:\n%s", output.Stderr())
 	}
 }
 
diff --git a/internal/command/testdata/test/mocking/tests/nested_mocked.tftest.hcl b/internal/command/testdata/test/mocking/tests/nested_mocked.tftest.hcl
new file mode 100644
index 000000000000..8c5925ac6210
--- /dev/null
+++ b/internal/command/testdata/test/mocking/tests/nested_mocked.tftest.hcl
@@ -0,0 +1,29 @@
+mock_provider "test" {
+  alias = "primary"
+
+  mock_resource "test_resource" {
+    defaults = {
+      id = "aaaa"
+      list_value = [{name  = "first"}, {name  = "second"}]
+      nested_list_value = [{name  = "first"}, {name  = "second"}]
+      set_value = [{name  = "first"}, {name  = "second"}]
+      nested_set_value = [{name  = "first"}, {name  = "second"}]
+      map_value = [{name  = "first"}, {name  = "second"}]
+      nested_map_value = [{name  = "first"}, {name  = "second"}]
+    }
+  }
+}
+
+variables {
+  instances = 1
+  child_instances = 0
+}
+
+
+run "test" {
+
+  assert {
+    condition = test_resource.primary[0].id == "aaaa"
+    error_message = "did not apply mocks"
+  }
+}
diff --git a/internal/command/testdata/test/override_data_list_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_attribute/list.tftest.hcl
similarity index 100%
rename from internal/command/testdata/test/override_data_list_attribute/main.tftest.hcl
rename to internal/command/testdata/test/override_data_attribute/list.tftest.hcl
diff --git a/internal/command/testdata/test/override_data_set_attribute/main.tf b/internal/command/testdata/test/override_data_attribute/main.tf
similarity index 100%
rename from internal/command/testdata/test/override_data_set_attribute/main.tf
rename to internal/command/testdata/test/override_data_attribute/main.tf
diff --git a/internal/command/testdata/test/override_data_map_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_attribute/map.tftest.hcl
similarity index 100%
rename from internal/command/testdata/test/override_data_map_attribute/main.tftest.hcl
rename to internal/command/testdata/test/override_data_attribute/map.tftest.hcl
diff --git a/internal/command/testdata/test/override_data_nested_list_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_attribute/nested_list.tftest.hcl
similarity index 100%
rename from internal/command/testdata/test/override_data_nested_list_attribute/main.tftest.hcl
rename to internal/command/testdata/test/override_data_attribute/nested_list.tftest.hcl
diff --git a/internal/command/testdata/test/override_data_nested_map_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_attribute/nested_map.tftest.hcl
similarity index 100%
rename from internal/command/testdata/test/override_data_nested_map_attribute/main.tftest.hcl
rename to internal/command/testdata/test/override_data_attribute/nested_map.tftest.hcl
diff --git a/internal/command/testdata/test/override_data_nested_set_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_attribute/nested_set.tftest.hcl
similarity index 100%
rename from internal/command/testdata/test/override_data_nested_set_attribute/main.tftest.hcl
rename to internal/command/testdata/test/override_data_attribute/nested_set.tftest.hcl
diff --git a/internal/command/testdata/test/override_data_set_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_attribute/set.tftest.hcl
similarity index 100%
rename from internal/command/testdata/test/override_data_set_attribute/main.tftest.hcl
rename to internal/command/testdata/test/override_data_attribute/set.tftest.hcl
diff --git a/internal/command/testdata/test/override_data_list_attribute/main.tf b/internal/command/testdata/test/override_data_list_attribute/main.tf
deleted file mode 100644
index 00330c09aea6..000000000000
--- a/internal/command/testdata/test/override_data_list_attribute/main.tf
+++ /dev/null
@@ -1,15 +0,0 @@
-terraform {
-  required_providers {
-    test = {
-      source = "hashicorp/test"
-    }
-  }
-}
-
-data "test_complex_data_source" "datasource" {
-  id = "resource"
-}
-
-output "list_value" {
-  value = data.test_complex_data_source.datasource.list_value
-}
diff --git a/internal/command/testdata/test/override_data_map_attribute/main.tf b/internal/command/testdata/test/override_data_map_attribute/main.tf
deleted file mode 100644
index 520c9e1de8bf..000000000000
--- a/internal/command/testdata/test/override_data_map_attribute/main.tf
+++ /dev/null
@@ -1,15 +0,0 @@
-terraform {
-  required_providers {
-    test = {
-      source = "hashicorp/test"
-    }
-  }
-}
-
-data "test_complex_data_source" "datasource" {
-  id = "resource"
-}
-
-output "map_value" {
-  value = data.test_complex_data_source.datasource.map_value
-}
diff --git a/internal/command/testdata/test/override_data_nested_list_attribute/main.tf b/internal/command/testdata/test/override_data_nested_list_attribute/main.tf
deleted file mode 100644
index ea41b36bc2e2..000000000000
--- a/internal/command/testdata/test/override_data_nested_list_attribute/main.tf
+++ /dev/null
@@ -1,15 +0,0 @@
-terraform {
-  required_providers {
-    test = {
-      source = "hashicorp/test"
-    }
-  }
-}
-
-data "test_complex_data_source" "datasource" {
-  id = "resource"
-}
-
-output "nested_list_value" {
-  value = data.test_complex_data_source.datasource.nested_list_value
-}
diff --git a/internal/command/testdata/test/override_data_nested_map_attribute/main.tf b/internal/command/testdata/test/override_data_nested_map_attribute/main.tf
deleted file mode 100644
index 0df44632df41..000000000000
--- a/internal/command/testdata/test/override_data_nested_map_attribute/main.tf
+++ /dev/null
@@ -1,15 +0,0 @@
-terraform {
-  required_providers {
-    test = {
-      source = "hashicorp/test"
-    }
-  }
-}
-
-data "test_complex_data_source" "datasource" {
-  id = "resource"
-}
-
-output "nested_map_value" {
-  value = data.test_complex_data_source.datasource.nested_map_value
-}
diff --git a/internal/command/testdata/test/override_data_nested_set_attribute/main.tf b/internal/command/testdata/test/override_data_nested_set_attribute/main.tf
deleted file mode 100644
index 5174b78b954f..000000000000
--- a/internal/command/testdata/test/override_data_nested_set_attribute/main.tf
+++ /dev/null
@@ -1,15 +0,0 @@
-terraform {
-  required_providers {
-    test = {
-      source = "hashicorp/test"
-    }
-  }
-}
-
-data "test_complex_data_source" "datasource" {
-  id = "resource"
-}
-
-output "nested_set_value" {
-  value = data.test_complex_data_source.datasource.nested_set_value
-}

From 2d5f51edd48cf620c39420927e643e253ec01cd1 Mon Sep 17 00:00:00 2001
From: Samsondeen Dare <samsondeen.dare@hashicorp.com>
Date: Mon, 23 Feb 2026 14:11:10 +0100
Subject: [PATCH 07/11] add test for mock_resource

---
 .../command/testdata/test/mocking/main.tf     |  11 +-
 .../mocking/tests/nested_mocked.tftest.hcl    |  42 ++++-
 internal/command/testing/test_provider.go     | 151 ++++++++++--------
 3 files changed, 133 insertions(+), 71 deletions(-)

diff --git a/internal/command/testdata/test/mocking/main.tf b/internal/command/testdata/test/mocking/main.tf
index 49506e06c38f..9d0180ca1b24 100644
--- a/internal/command/testdata/test/mocking/main.tf
+++ b/internal/command/testdata/test/mocking/main.tf
@@ -24,12 +24,17 @@ variable "child_instances" {
 
 resource "test_resource" "primary" {
   provider = test.primary
-  count = var.instances
+  count    = var.instances
+}
+
+resource "test_complex_resource" "primary" {
+  provider = test.primary
+  count    = var.instances
 }
 
 resource "test_resource" "secondary" {
   provider = test.secondary
-  count = var.instances
+  count    = var.instances
 }
 
 module "child" {
@@ -38,7 +43,7 @@ module "child" {
   source = "./child"
 
   providers = {
-    test.primary = test.primary
+    test.primary   = test.primary
     test.secondary = test.secondary
   }
 
diff --git a/internal/command/testdata/test/mocking/tests/nested_mocked.tftest.hcl b/internal/command/testdata/test/mocking/tests/nested_mocked.tftest.hcl
index 8c5925ac6210..28cb1d9be769 100644
--- a/internal/command/testdata/test/mocking/tests/nested_mocked.tftest.hcl
+++ b/internal/command/testdata/test/mocking/tests/nested_mocked.tftest.hcl
@@ -1,15 +1,29 @@
 mock_provider "test" {
   alias = "primary"
 
-  mock_resource "test_resource" {
+  mock_resource "test_complex_resource" {
     defaults = {
       id = "aaaa"
       list_value = [{name  = "first"}, {name  = "second"}]
       nested_list_value = [{name  = "first"}, {name  = "second"}]
       set_value = [{name  = "first"}, {name  = "second"}]
       nested_set_value = [{name  = "first"}, {name  = "second"}]
-      map_value = [{name  = "first"}, {name  = "second"}]
-      nested_map_value = [{name  = "first"}, {name  = "second"}]
+      map_value = {
+        "key1": {
+          name  = "first"
+        },
+        "key2": {
+          name  = "third"
+        }
+      }
+      nested_map_value = {
+        "key1": {
+          name  = "first"
+        },
+        "key2": {
+          name  = "third"
+        }
+      }
     }
   }
 }
@@ -23,7 +37,27 @@ variables {
 run "test" {
 
   assert {
-    condition = test_resource.primary[0].id == "aaaa"
+    condition = test_complex_resource.primary[0].id == "aaaa"
+    error_message = "did not apply mocks"
+  }
+  
+  assert {
+    condition = test_complex_resource.primary[0].list_value[0].name == "first"
+    error_message = "did not apply mocks"
+  }
+  
+  assert {
+    condition = test_complex_resource.primary[0].nested_list_value[0].name == "first"
+    error_message = "did not apply mocks"
+  }
+  
+  assert {
+    condition = test_complex_resource.primary[0].map_value["key1"].name == "first"
+    error_message = "did not apply mocks"
+  }
+  
+  assert {
+    condition = test_complex_resource.primary[0].nested_map_value["key1"].name == "first"
     error_message = "did not apply mocks"
   }
 }
diff --git a/internal/command/testing/test_provider.go b/internal/command/testing/test_provider.go
index 2f921975b3db..3feda5ea9be4 100644
--- a/internal/command/testing/test_provider.go
+++ b/internal/command/testing/test_provider.go
@@ -5,6 +5,7 @@ package testing
 
 import (
 	"fmt"
+	"maps"
 	"path"
 	"strings"
 	"sync"
@@ -20,6 +21,76 @@ import (
 )
 
 var (
+	// withBlockTypeAttributes contains additional block type attributes that are used across
+	// different resources and types, so this function serves as a place to store the shared attributes.
+	withBlockTypeAttributes = func(original map[string]*configschema.Attribute) map[string]*configschema.Attribute {
+		attrs := map[string]*configschema.Attribute{
+			"list_value": {
+				Type: cty.List(cty.Object(map[string]cty.Type{
+					"name":  cty.String,
+					"value": cty.String,
+				})),
+				Computed: true,
+				Optional: true,
+			},
+			"set_value": {
+				Type: cty.Set(cty.Object(map[string]cty.Type{
+					"name":  cty.String,
+					"value": cty.String,
+				})),
+				Computed: true,
+				Optional: true,
+			},
+			"map_value": {
+				Type: cty.Map(cty.Object(map[string]cty.Type{
+					"name":  cty.String,
+					"value": cty.String,
+				})),
+				Computed: true,
+				Optional: true,
+			},
+
+			// The below nested_* attributes represent config supporting
+			// attributes as nested blocks, where terraform interprets the nested blocks
+			// as attributes within the schema
+			"nested_list_value": {
+				Computed: true,
+				Optional: true,
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingList,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String, Optional: true},
+						"value": {Type: cty.String, Optional: true},
+					},
+				},
+			},
+			"nested_set_value": {
+				Computed: true,
+				Optional: true,
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingSet,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String, Optional: true},
+						"value": {Type: cty.String, Optional: true},
+					},
+				},
+			},
+			"nested_map_value": {
+				Computed: true,
+				Optional: true,
+				NestedType: &configschema.Object{
+					Nesting: configschema.NestingMap,
+					Attributes: map[string]*configschema.Attribute{
+						"name":  {Type: cty.String, Optional: true},
+						"value": {Type: cty.String, Optional: true},
+					},
+				},
+			},
+		}
+		maps.Copy(original, attrs)
+		return original
+	}
+
 	ProviderSchema = &providers.GetProviderSchemaResponse{
 		Provider: providers.Schema{
 			Body: &configschema.Block{
@@ -44,6 +115,20 @@ var (
 					},
 				},
 			},
+			"test_complex_resource": {
+				Body: &configschema.Block{
+					Attributes: withBlockTypeAttributes(map[string]*configschema.Attribute{
+						"id":                   {Type: cty.String, Optional: true, Computed: true},
+						"value":                {Type: cty.String, Optional: true},
+						"interrupt_count":      {Type: cty.Number, Optional: true},
+						"destroy_fail":         {Type: cty.Bool, Optional: true, Computed: true},
+						"create_wait_seconds":  {Type: cty.Number, Optional: true},
+						"destroy_wait_seconds": {Type: cty.Number, Optional: true},
+						"write_only":           {Type: cty.String, Optional: true, WriteOnly: true},
+						"defer":                {Type: cty.Bool, Optional: true},
+					}),
+				},
+			},
 		},
 		DataSources: map[string]providers.Schema{
 			"test_data_source": {
@@ -69,73 +154,11 @@ var (
 
 			"test_complex_data_source": {
 				Body: &configschema.Block{
-					Attributes: map[string]*configschema.Attribute{
+					Attributes: withBlockTypeAttributes(map[string]*configschema.Attribute{
 						"id":         {Type: cty.String, Required: true},
 						"value":      {Type: cty.String, Computed: true},
 						"write_only": {Type: cty.String, Optional: true, WriteOnly: true},
 
-						"list_value": {
-							Type: cty.List(cty.Object(map[string]cty.Type{
-								"name":  cty.String,
-								"value": cty.String,
-							})),
-							Computed: true,
-							Optional: true,
-						},
-						"set_value": {
-							Type: cty.Set(cty.Object(map[string]cty.Type{
-								"name":  cty.String,
-								"value": cty.String,
-							})),
-							Computed: true,
-							Optional: true,
-						},
-						"map_value": {
-							Type: cty.Map(cty.Object(map[string]cty.Type{
-								"name":  cty.String,
-								"value": cty.String,
-							})),
-							Computed: true,
-							Optional: true,
-						},
-
-						// The below nested_* attributes represent config supporting
-						// attributes as nested blocks, where terraform interprets the nested blocks
-						// as attributes within the schema
-						"nested_list_value": {
-							Computed: true,
-							Optional: true,
-							NestedType: &configschema.Object{
-								Nesting: configschema.NestingList,
-								Attributes: map[string]*configschema.Attribute{
-									"name":  {Type: cty.String, Optional: true},
-									"value": {Type: cty.String, Optional: true},
-								},
-							},
-						},
-						"nested_set_value": {
-							Computed: true,
-							Optional: true,
-							NestedType: &configschema.Object{
-								Nesting: configschema.NestingSet,
-								Attributes: map[string]*configschema.Attribute{
-									"name":  {Type: cty.String, Optional: true},
-									"value": {Type: cty.String, Optional: true},
-								},
-							},
-						},
-						"nested_map_value": {
-							Computed: true,
-							Optional: true,
-							NestedType: &configschema.Object{
-								Nesting: configschema.NestingMap,
-								Attributes: map[string]*configschema.Attribute{
-									"name":  {Type: cty.String, Optional: true},
-									"value": {Type: cty.String, Optional: true},
-								},
-							},
-						},
-
 						// We never actually reference these values from a data
 						// source, but we have tests that use the same cty.Value
 						// to represent a test_resource and a test_data_source
@@ -146,7 +169,7 @@ var (
 						"create_wait_seconds":  {Type: cty.Number, Computed: true},
 						"destroy_wait_seconds": {Type: cty.Number, Computed: true},
 						"defer":                {Type: cty.Bool, Computed: true},
-					},
+					}),
 				},
 			},
 		},

From 76440932801161b3f2278356bbe2f88d2e844056 Mon Sep 17 00:00:00 2001
From: Samsondeen Dare <samsondeen.dare@hashicorp.com>
Date: Mon, 23 Feb 2026 10:43:39 +0100
Subject: [PATCH 08/11] Fill computed values of Nested type

---
 internal/command/test_test.go                 | 109 +++++++++++-------
 .../test/override_data_list_attribute/main.tf |  15 +++
 .../main.tftest.hcl                           |  46 ++++++++
 .../main.tf                                   |  15 +++
 .../main.tftest.hcl                           |  46 ++++++++
 internal/command/testing/test_provider.go     |  27 +++++
 internal/moduletest/mocking/fill.go           |  57 ++++++++-
 7 files changed, 271 insertions(+), 44 deletions(-)
 create mode 100644 internal/command/testdata/test/override_data_list_attribute/main.tf
 create mode 100644 internal/command/testdata/test/override_data_list_attribute/main.tftest.hcl
 create mode 100644 internal/command/testdata/test/override_data_nested_list_attribute/main.tf
 create mode 100644 internal/command/testdata/test/override_data_nested_list_attribute/main.tftest.hcl

diff --git a/internal/command/test_test.go b/internal/command/test_test.go
index e163db8380f1..ee7292b5c223 100644
--- a/internal/command/test_test.go
+++ b/internal/command/test_test.go
@@ -5674,59 +5674,82 @@ func TestTest_TeardownOrder(t *testing.T) {
 	}
 }
 
-func TestTest_OverrideData(t *testing.T) {
-	td := t.TempDir()
-	testCopyDir(t, testFixturePath(path.Join("test", "override_data_attribute")), td)
-	t.Chdir(td)
+func TestTest_OverrideDataListAttribute(t *testing.T) {
+	tcs := map[string]struct {
+		dir  string
+		code int
+		desc string
+	}{
+		"plain_list_attribute": {
+			dir:  "override_data_list_attribute",
+			code: 0,
+			desc: "override_data with a computed cty.List(cty.Object) attribute",
+		},
+		"nested_list_attribute": {
+			dir:  "override_data_nested_list_attribute",
+			code: 0,
+			desc: "override_data with a computed NestedType NestingList attribute",
+		},
+	}
 
-	provider := testing_command.NewProvider(nil)
-	providerSource, closeFn := newMockProviderSource(t, map[string][]string{
-		"test": {"1.0.0"},
-	})
-	defer closeFn()
+	for name, tc := range tcs {
+		t.Run(name, func(t *testing.T) {
+			td := t.TempDir()
+			testCopyDir(t, testFixturePath(path.Join("test", tc.dir)), td)
+			t.Chdir(td)
 
-	streams, done := terminal.StreamsForTesting(t)
-	view := views.NewView(streams)
-	ui := new(cli.MockUi)
+			provider := testing_command.NewProvider(nil)
+			providerSource, closeFn := newMockProviderSource(t, map[string][]string{
+				"test": {"1.0.0"},
+			})
+			defer closeFn()
 
-	meta := Meta{
-		testingOverrides: metaOverridesForProvider(provider.Provider),
-		Ui:               ui,
-		View:             view,
-		Streams:          streams,
-		ProviderSource:   providerSource,
-	}
+			streams, done := terminal.StreamsForTesting(t)
+			view := views.NewView(streams)
+			ui := new(cli.MockUi)
 
-	init := &InitCommand{
-		Meta: meta,
-	}
+			meta := Meta{
+				testingOverrides: metaOverridesForProvider(provider.Provider),
+				Ui:               ui,
+				View:             view,
+				Streams:          streams,
+				ProviderSource:   providerSource,
+			}
 
-	if code := init.Run(nil); code != 0 {
-		output := done(t)
-		t.Fatalf("expected init status code 0 but got %d: %s", code, output.All())
-	}
+			init := &InitCommand{
+				Meta: meta,
+			}
 
-	// Reset the streams for the next command.
-	streams, done = terminal.StreamsForTesting(t)
-	meta.Streams = streams
-	meta.View = views.NewView(streams)
+			if code := init.Run(nil); code != 0 {
+				output := done(t)
+				t.Fatalf("expected init status code 0 but got %d: %s", code, output.All())
+			}
 
-	c := &TestCommand{
-		Meta: meta,
-	}
+			// Reset the streams for the next command.
+			streams, done = terminal.StreamsForTesting(t)
+			meta.Streams = streams
+			meta.View = views.NewView(streams)
 
-	code := c.Run([]string{"-no-color"})
-	output := done(t)
+			c := &TestCommand{
+				Meta: meta,
+			}
 
-	if code != 0 {
-		t.Errorf("expected status code %d but got %d:\n\n%s", 0, code, output.All())
-	}
+			code := c.Run([]string{"-no-color"})
+			output := done(t)
 
-	if !strings.Contains(output.Stdout(), "6 passed, 0 failed.") {
-		t.Errorf("expected passing test output but got:\n\nstdout:\n%s\nstderr:\n%s", output.Stdout(), output.Stderr())
-	}
-	if output.Stderr() != "" {
-		t.Errorf("unexpected stderr output:\n%s", output.Stderr())
+			if code != tc.code {
+				t.Errorf("expected status code %d but got %d:\n\n%s", tc.code, code, output.All())
+			}
+
+			if tc.code == 0 {
+				if !strings.Contains(output.Stdout(), "1 passed, 0 failed.") {
+					t.Errorf("expected passing test output but got:\n\nstdout:\n%s\nstderr:\n%s", output.Stdout(), output.Stderr())
+				}
+				if output.Stderr() != "" {
+					t.Errorf("unexpected stderr output:\n%s", output.Stderr())
+				}
+			}
+		})
 	}
 }
 
diff --git a/internal/command/testdata/test/override_data_list_attribute/main.tf b/internal/command/testdata/test/override_data_list_attribute/main.tf
new file mode 100644
index 000000000000..f9d19bfd7435
--- /dev/null
+++ b/internal/command/testdata/test/override_data_list_attribute/main.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    test = {
+      source = "hashicorp/test"
+    }
+  }
+}
+
+data "test_data_source" "datasource" {
+  id = "resource"
+}
+
+output "list_value" {
+  value = data.test_data_source.datasource.list_value
+}
diff --git a/internal/command/testdata/test/override_data_list_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_list_attribute/main.tftest.hcl
new file mode 100644
index 000000000000..597e98ac4f9f
--- /dev/null
+++ b/internal/command/testdata/test/override_data_list_attribute/main.tftest.hcl
@@ -0,0 +1,46 @@
+provider "test" {}
+
+override_data {
+  target = data.test_data_source.datasource
+  values = {
+    list_value = [
+      {
+        name  = "first"
+        value = "one"
+      },
+      {
+        name  = "second"
+        value = "two"
+      },
+    ]
+  }
+}
+
+run "test_override_data_list_attribute" {
+  command = plan
+
+  assert {
+    condition     = length(data.test_data_source.datasource.list_value) == 2
+    error_message = "Expected list_value to have 2 elements, got ${length(data.test_data_source.datasource.list_value)}"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.list_value[0].name == "first"
+    error_message = "Expected first element name to be 'first'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.list_value[0].value == "one"
+    error_message = "Expected first element value to be 'one'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.list_value[1].name == "second"
+    error_message = "Expected second element name to be 'second'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.list_value[1].value == "two"
+    error_message = "Expected second element value to be 'two'"
+  }
+}
diff --git a/internal/command/testdata/test/override_data_nested_list_attribute/main.tf b/internal/command/testdata/test/override_data_nested_list_attribute/main.tf
new file mode 100644
index 000000000000..52cfeb5fa44a
--- /dev/null
+++ b/internal/command/testdata/test/override_data_nested_list_attribute/main.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    test = {
+      source = "hashicorp/test"
+    }
+  }
+}
+
+data "test_data_source" "datasource" {
+  id = "resource"
+}
+
+output "nested_list_value" {
+  value = data.test_data_source.datasource.nested_list_value
+}
diff --git a/internal/command/testdata/test/override_data_nested_list_attribute/main.tftest.hcl b/internal/command/testdata/test/override_data_nested_list_attribute/main.tftest.hcl
new file mode 100644
index 000000000000..fe5a85019600
--- /dev/null
+++ b/internal/command/testdata/test/override_data_nested_list_attribute/main.tftest.hcl
@@ -0,0 +1,46 @@
+provider "test" {}
+
+override_data {
+  target = data.test_data_source.datasource
+  values = {
+    nested_list_value = [
+      {
+        name  = "first"
+        value = "one"
+      },
+      {
+        name  = "second"
+        value = "two"
+      },
+    ]
+  }
+}
+
+run "test_override_data_nested_list_attribute" {
+  command = plan
+
+  assert {
+    condition     = length(data.test_data_source.datasource.nested_list_value) == 2
+    error_message = "Expected nested_list_value to have 2 elements, got ${length(data.test_data_source.datasource.nested_list_value)}"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.nested_list_value[0].name == "first"
+    error_message = "Expected first element name to be 'first'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.nested_list_value[0].value == "one"
+    error_message = "Expected first element value to be 'one'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.nested_list_value[1].name == "second"
+    error_message = "Expected second element name to be 'second'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.nested_list_value[1].value == "two"
+    error_message = "Expected second element value to be 'two'"
+  }
+}
diff --git a/internal/command/testing/test_provider.go b/internal/command/testing/test_provider.go
index 3feda5ea9be4..a2c3390fb2f9 100644
--- a/internal/command/testing/test_provider.go
+++ b/internal/command/testing/test_provider.go
@@ -138,6 +138,33 @@ var (
 						"value":      {Type: cty.String, Computed: true},
 						"write_only": {Type: cty.String, Optional: true, WriteOnly: true},
 
+						// list_value is a computed attribute of list-of-objects
+						// type, used to test that override_data can correctly
+						// override list-type attributes (see GitHub issue #37939).
+						"list_value": {
+							Type: cty.List(cty.Object(map[string]cty.Type{
+								"name":  cty.String,
+								"value": cty.String,
+							})),
+							Computed: true,
+						},
+
+						// nested_list_value is a computed attribute using NestedType
+						// with NestingList, used to test that override_data can correctly
+						// override nested list-type attributes (see GitHub issue #37939).
+						// Many real-world providers (e.g. Databricks) define list-of-objects
+						// attributes this way rather than using a plain cty.List type.
+						"nested_list_value": {
+							Computed: true,
+							NestedType: &configschema.Object{
+								Nesting: configschema.NestingList,
+								Attributes: map[string]*configschema.Attribute{
+									"name":  {Type: cty.String, Optional: true},
+									"value": {Type: cty.String, Optional: true},
+								},
+							},
+						},
+
 						// We never actually reference these values from a data
 						// source, but we have tests that use the same cty.Value
 						// to represent a test_resource and a test_data_source
diff --git a/internal/moduletest/mocking/fill.go b/internal/moduletest/mocking/fill.go
index 469e2735378b..6eccf33e214d 100644
--- a/internal/moduletest/mocking/fill.go
+++ b/internal/moduletest/mocking/fill.go
@@ -24,12 +24,67 @@ func FillAttribute(in cty.Value, attribute *configschema.Attribute) (cty.Value,
 func fillAttribute(in cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
 	ty := attribute.Type
 	if attribute.NestedType != nil {
-		ty = attribute.NestedType.ConfigType()
+
+		switch attribute.NestedType.Nesting {
+		case configschema.NestingSingle, configschema.NestingGroup:
+			return fillObject(in, attribute, path)
+		case configschema.NestingSet:
+			return cty.SetValEmpty(attribute.ImpliedType().ElementType()), nil
+		case configschema.NestingList:
+			return fillIterable(in, attribute, path)
+		case configschema.NestingMap:
+			return cty.MapValEmpty(attribute.ImpliedType().ElementType()), nil
+		default:
+			panic(fmt.Errorf("unknown nesting mode: %d", attribute.NestedType.Nesting))
+		}
 	}
 
 	return fillType(in, ty, path)
 }
 
+func fillObject(in cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
+	// Then the in value must be an object.
+	if !in.Type().IsObjectType() {
+		return cty.NilVal, path.NewErrorf("incompatible types; expected object type, found %s", in.Type().FriendlyName())
+	}
+
+	var names []string
+	for name := range attribute.NestedType.Attributes {
+		names = append(names, name)
+	}
+	if len(names) == 0 {
+		return cty.EmptyObjectVal, nil
+	}
+
+	// Make the order we iterate through the attributes deterministic. We
+	// are generating random strings in here so it's worth making the
+	// operation repeatable.
+	sort.Strings(names)
+
+	children := make(map[string]cty.Value)
+	for _, name := range names {
+		if in.Type().HasAttribute(name) {
+			child, err := fillAttribute(in.GetAttr(name), attribute.NestedType.Attributes[name], path.GetAttr(name))
+			if err != nil {
+				return cty.NilVal, err
+			}
+			children[name] = child
+			continue
+		}
+		children[name] = GenerateValueForAttribute(attribute.NestedType.Attributes[name])
+	}
+	return cty.ObjectVal(children), nil
+}
+
+func fillIterable(in cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
+	ty := attribute.NestedType.ConfigType()
+	out, err := fillType(in, ty, path)
+	if err != nil {
+		return cty.NilVal, err
+	}
+	return out, err
+}
+
 // FillType makes the input value match the target type by adding attributes
 // directly to it or to any nested objects. Essentially, this is a "safe"
 // conversion between two objects.

From da4cf6b5c1ad1352d5e92abfcb5951b38aba61cb Mon Sep 17 00:00:00 2001
From: Samsondeen Dare <samsondeen.dare@hashicorp.com>
Date: Mon, 13 Apr 2026 12:57:53 +0200
Subject: [PATCH 09/11] retain legacy behaviour of empty collection when given
 an object as mock

---
 internal/command/test_test.go                 | 73 ++++++++++++++-----
 .../main.tf                                   | 15 ++++
 .../main.tftest.hcl                           | 19 +++++
 .../main.tf                                   | 15 ++++
 .../main.tftest.hcl                           | 40 ++++++++++
 .../main.tf                                   | 15 ++++
 .../main.tftest.hcl                           | 44 +++++++++++
 .../main.tf                                   | 15 ++++
 .../main.tftest.hcl                           | 12 +++
 .../main.tf                                   | 15 ++++
 .../main.tftest.hcl                           | 19 +++++
 .../main.tf                                   | 15 ++++
 .../main.tftest.hcl                           | 44 +++++++++++
 internal/moduletest/mocking/fill.go           | 47 ++++++++----
 14 files changed, 352 insertions(+), 36 deletions(-)
 create mode 100644 internal/command/testdata/test/override_data_complex_nested_set_attribute_object/main.tf
 create mode 100644 internal/command/testdata/test/override_data_complex_nested_set_attribute_object/main.tftest.hcl
 create mode 100644 internal/command/testdata/test/override_data_complex_set_attribute_partial_elements/main.tf
 create mode 100644 internal/command/testdata/test/override_data_complex_set_attribute_partial_elements/main.tftest.hcl
 create mode 100644 internal/command/testdata/test/override_data_list_attribute_partial_elements/main.tf
 create mode 100644 internal/command/testdata/test/override_data_list_attribute_partial_elements/main.tftest.hcl
 create mode 100644 internal/command/testdata/test/override_data_nested_list_attribute_invalid_type/main.tf
 create mode 100644 internal/command/testdata/test/override_data_nested_list_attribute_invalid_type/main.tftest.hcl
 create mode 100644 internal/command/testdata/test/override_data_nested_list_attribute_object/main.tf
 create mode 100644 internal/command/testdata/test/override_data_nested_list_attribute_object/main.tftest.hcl
 create mode 100644 internal/command/testdata/test/override_data_nested_list_attribute_partial_elements/main.tf
 create mode 100644 internal/command/testdata/test/override_data_nested_list_attribute_partial_elements/main.tftest.hcl

diff --git a/internal/command/test_test.go b/internal/command/test_test.go
index ee7292b5c223..d96840e39aa8 100644
--- a/internal/command/test_test.go
+++ b/internal/command/test_test.go
@@ -5674,21 +5674,52 @@ func TestTest_TeardownOrder(t *testing.T) {
 	}
 }
 
-func TestTest_OverrideDataListAttribute(t *testing.T) {
+func TestTest_OverrideDataMocking(t *testing.T) {
 	tcs := map[string]struct {
-		dir  string
-		code int
-		desc string
+		dir            string
+		expectedCode   int
+		expectedStdout string
+		expectedStderr string
 	}{
 		"plain_list_attribute": {
-			dir:  "override_data_list_attribute",
-			code: 0,
-			desc: "override_data with a computed cty.List(cty.Object) attribute",
+			dir:            "override_data_list_attribute",
+			expectedCode:   0,
+			expectedStdout: "1 passed, 0 failed.",
 		},
 		"nested_list_attribute": {
-			dir:  "override_data_nested_list_attribute",
-			code: 0,
-			desc: "override_data with a computed NestedType NestingList attribute",
+			dir:            "override_data_nested_list_attribute",
+			expectedCode:   0,
+			expectedStdout: "1 passed, 0 failed.",
+		},
+		"nested_list_attribute_with_object_value": {
+			dir:            "override_data_nested_list_attribute_object",
+			expectedCode:   0,
+			expectedStdout: "1 passed, 0 failed.",
+		},
+		"nested_list_attribute_with_invalid_type_value": {
+			dir:            "override_data_nested_list_attribute_invalid_type",
+			expectedCode:   1,
+			expectedStderr: "incompatible types; expected list of object, found",
+		},
+		"list_attribute_with_partial_element_values": {
+			dir:            "override_data_list_attribute_partial_elements",
+			expectedCode:   0,
+			expectedStdout: "1 passed, 0 failed.",
+		},
+		"set_attribute_with_partial_element_values": {
+			dir:            "override_data_complex_set_attribute_partial_elements",
+			expectedCode:   0,
+			expectedStdout: "1 passed, 0 failed.",
+		},
+		"nested_set_attribute_with_object_value": {
+			dir:            "override_data_complex_nested_set_attribute_object",
+			expectedCode:   0,
+			expectedStdout: "1 passed, 0 failed.",
+		},
+		"nested_list_attribute_with_partial_element_values": {
+			dir:            "override_data_nested_list_attribute_partial_elements",
+			expectedCode:   0,
+			expectedStdout: "1 passed, 0 failed.",
 		},
 	}
 
@@ -5725,7 +5756,6 @@ func TestTest_OverrideDataListAttribute(t *testing.T) {
 				t.Fatalf("expected init status code 0 but got %d: %s", code, output.All())
 			}
 
-			// Reset the streams for the next command.
 			streams, done = terminal.StreamsForTesting(t)
 			meta.Streams = streams
 			meta.View = views.NewView(streams)
@@ -5737,17 +5767,20 @@ func TestTest_OverrideDataListAttribute(t *testing.T) {
 			code := c.Run([]string{"-no-color"})
 			output := done(t)
 
-			if code != tc.code {
-				t.Errorf("expected status code %d but got %d:\n\n%s", tc.code, code, output.All())
+			if code != tc.expectedCode {
+				t.Fatalf("expected status code %d but got %d:\n\n%s", tc.expectedCode, code, output.All())
 			}
 
-			if tc.code == 0 {
-				if !strings.Contains(output.Stdout(), "1 passed, 0 failed.") {
-					t.Errorf("expected passing test output but got:\n\nstdout:\n%s\nstderr:\n%s", output.Stdout(), output.Stderr())
-				}
-				if output.Stderr() != "" {
-					t.Errorf("unexpected stderr output:\n%s", output.Stderr())
-				}
+			if tc.expectedStdout != "" && !strings.Contains(output.Stdout(), tc.expectedStdout) {
+				t.Errorf("expected stdout to contain %q but got:\n\nstdout:\n%s\nstderr:\n%s", tc.expectedStdout, output.Stdout(), output.Stderr())
+			}
+
+			if tc.expectedStderr != "" && !strings.Contains(output.Stderr(), tc.expectedStderr) {
+				t.Errorf("expected stderr to contain %q but got:\n\nstdout:\n%s\nstderr:\n%s", tc.expectedStderr, output.Stdout(), output.Stderr())
+			}
+
+			if tc.expectedCode == 0 && output.Stderr() != "" {
+				t.Errorf("unexpected stderr output:\n%s", output.Stderr())
 			}
 		})
 	}
diff --git a/internal/command/testdata/test/override_data_complex_nested_set_attribute_object/main.tf b/internal/command/testdata/test/override_data_complex_nested_set_attribute_object/main.tf
new file mode 100644
index 000000000000..5174b78b954f
--- /dev/null
+++ b/internal/command/testdata/test/override_data_complex_nested_set_attribute_object/main.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    test = {
+      source = "hashicorp/test"
+    }
+  }
+}
+
+data "test_complex_data_source" "datasource" {
+  id = "resource"
+}
+
+output "nested_set_value" {
+  value = data.test_complex_data_source.datasource.nested_set_value
+}
diff --git a/internal/command/testdata/test/override_data_complex_nested_set_attribute_object/main.tftest.hcl b/internal/command/testdata/test/override_data_complex_nested_set_attribute_object/main.tftest.hcl
new file mode 100644
index 000000000000..f18d29348530
--- /dev/null
+++ b/internal/command/testdata/test/override_data_complex_nested_set_attribute_object/main.tftest.hcl
@@ -0,0 +1,19 @@
+provider "test" {}
+
+override_data {
+  target = data.test_complex_data_source.datasource
+  values = {
+    nested_set_value = {
+      name = "shared"
+    }
+  }
+}
+
+run "test_override_data_complex_nested_set_attribute_object" {
+  command = plan
+
+  assert {
+    condition     = length(data.test_complex_data_source.datasource.nested_set_value) == 0
+    error_message = "Expected nested_set_value to be empty when overridden with an object"
+  }
+}
diff --git a/internal/command/testdata/test/override_data_complex_set_attribute_partial_elements/main.tf b/internal/command/testdata/test/override_data_complex_set_attribute_partial_elements/main.tf
new file mode 100644
index 000000000000..bbea87bd356d
--- /dev/null
+++ b/internal/command/testdata/test/override_data_complex_set_attribute_partial_elements/main.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    test = {
+      source = "hashicorp/test"
+    }
+  }
+}
+
+data "test_complex_data_source" "datasource" {
+  id = "resource"
+}
+
+output "set_value" {
+  value = data.test_complex_data_source.datasource.set_value
+}
diff --git a/internal/command/testdata/test/override_data_complex_set_attribute_partial_elements/main.tftest.hcl b/internal/command/testdata/test/override_data_complex_set_attribute_partial_elements/main.tftest.hcl
new file mode 100644
index 000000000000..14f89f2de39c
--- /dev/null
+++ b/internal/command/testdata/test/override_data_complex_set_attribute_partial_elements/main.tftest.hcl
@@ -0,0 +1,40 @@
+provider "test" {}
+
+override_data {
+  target = data.test_complex_data_source.datasource
+  values = {
+    set_value = [
+      {
+        name = "first"
+      },
+      {
+        value = "two"
+      },
+    ]
+  }
+}
+
+run "test_override_data_complex_set_attribute_partial_elements" {
+  command = plan
+
+  assert {
+    condition     = length(data.test_complex_data_source.datasource.set_value) == 2
+    error_message = "Expected set_value to have 2 elements, got ${length(data.test_complex_data_source.datasource.set_value)}"
+  }
+
+  assert {
+    condition = length([
+      for item in data.test_complex_data_source.datasource.set_value : item
+      if item.name == "first" && item.value != null
+    ]) == 1
+    error_message = "Expected one set_value element with name 'first' and a filled-in value"
+  }
+
+  assert {
+    condition = length([
+      for item in data.test_complex_data_source.datasource.set_value : item
+      if item.value == "two" && item.name != null
+    ]) == 1
+    error_message = "Expected one set_value element with value 'two' and a filled-in name"
+  }
+}
diff --git a/internal/command/testdata/test/override_data_list_attribute_partial_elements/main.tf b/internal/command/testdata/test/override_data_list_attribute_partial_elements/main.tf
new file mode 100644
index 000000000000..f9d19bfd7435
--- /dev/null
+++ b/internal/command/testdata/test/override_data_list_attribute_partial_elements/main.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    test = {
+      source = "hashicorp/test"
+    }
+  }
+}
+
+data "test_data_source" "datasource" {
+  id = "resource"
+}
+
+output "list_value" {
+  value = data.test_data_source.datasource.list_value
+}
diff --git a/internal/command/testdata/test/override_data_list_attribute_partial_elements/main.tftest.hcl b/internal/command/testdata/test/override_data_list_attribute_partial_elements/main.tftest.hcl
new file mode 100644
index 000000000000..66bc4929342a
--- /dev/null
+++ b/internal/command/testdata/test/override_data_list_attribute_partial_elements/main.tftest.hcl
@@ -0,0 +1,44 @@
+provider "test" {}
+
+override_data {
+  target = data.test_data_source.datasource
+  values = {
+    list_value = [
+      {
+        name = "first"
+      },
+      {
+        value = "two"
+      },
+    ]
+  }
+}
+
+run "test_override_data_list_attribute_partial_elements" {
+  command = plan
+
+  assert {
+    condition     = length(data.test_data_source.datasource.list_value) == 2
+    error_message = "Expected list_value to have 2 elements, got ${length(data.test_data_source.datasource.list_value)}"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.list_value[0].name == "first"
+    error_message = "Expected first element name to be 'first'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.list_value[0].value != null
+    error_message = "Expected first element value to be filled in"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.list_value[1].value == "two"
+    error_message = "Expected second element value to be 'two'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.list_value[1].name != null
+    error_message = "Expected second element name to be filled in"
+  }
+}
diff --git a/internal/command/testdata/test/override_data_nested_list_attribute_invalid_type/main.tf b/internal/command/testdata/test/override_data_nested_list_attribute_invalid_type/main.tf
new file mode 100644
index 000000000000..52cfeb5fa44a
--- /dev/null
+++ b/internal/command/testdata/test/override_data_nested_list_attribute_invalid_type/main.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    test = {
+      source = "hashicorp/test"
+    }
+  }
+}
+
+data "test_data_source" "datasource" {
+  id = "resource"
+}
+
+output "nested_list_value" {
+  value = data.test_data_source.datasource.nested_list_value
+}
diff --git a/internal/command/testdata/test/override_data_nested_list_attribute_invalid_type/main.tftest.hcl b/internal/command/testdata/test/override_data_nested_list_attribute_invalid_type/main.tftest.hcl
new file mode 100644
index 000000000000..cb3ffaee8884
--- /dev/null
+++ b/internal/command/testdata/test/override_data_nested_list_attribute_invalid_type/main.tftest.hcl
@@ -0,0 +1,12 @@
+provider "test" {}
+
+override_data {
+  target = data.test_data_source.datasource
+  values = {
+    nested_list_value = "wrong type"
+  }
+}
+
+run "test_override_data_nested_list_attribute_invalid_type" {
+  command = plan
+}
diff --git a/internal/command/testdata/test/override_data_nested_list_attribute_object/main.tf b/internal/command/testdata/test/override_data_nested_list_attribute_object/main.tf
new file mode 100644
index 000000000000..52cfeb5fa44a
--- /dev/null
+++ b/internal/command/testdata/test/override_data_nested_list_attribute_object/main.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    test = {
+      source = "hashicorp/test"
+    }
+  }
+}
+
+data "test_data_source" "datasource" {
+  id = "resource"
+}
+
+output "nested_list_value" {
+  value = data.test_data_source.datasource.nested_list_value
+}
diff --git a/internal/command/testdata/test/override_data_nested_list_attribute_object/main.tftest.hcl b/internal/command/testdata/test/override_data_nested_list_attribute_object/main.tftest.hcl
new file mode 100644
index 000000000000..125cbc3b94c0
--- /dev/null
+++ b/internal/command/testdata/test/override_data_nested_list_attribute_object/main.tftest.hcl
@@ -0,0 +1,19 @@
+provider "test" {}
+
+override_data {
+  target = data.test_data_source.datasource
+  values = {
+    nested_list_value = {
+      name = "shared"
+    }
+  }
+}
+
+run "test_override_data_nested_list_attribute_object" {
+  command = plan
+
+  assert {
+    condition     = length(data.test_data_source.datasource.nested_list_value) == 0
+    error_message = "Expected nested_list_value to be empty when overridden with an object"
+  }
+}
diff --git a/internal/command/testdata/test/override_data_nested_list_attribute_partial_elements/main.tf b/internal/command/testdata/test/override_data_nested_list_attribute_partial_elements/main.tf
new file mode 100644
index 000000000000..52cfeb5fa44a
--- /dev/null
+++ b/internal/command/testdata/test/override_data_nested_list_attribute_partial_elements/main.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    test = {
+      source = "hashicorp/test"
+    }
+  }
+}
+
+data "test_data_source" "datasource" {
+  id = "resource"
+}
+
+output "nested_list_value" {
+  value = data.test_data_source.datasource.nested_list_value
+}
diff --git a/internal/command/testdata/test/override_data_nested_list_attribute_partial_elements/main.tftest.hcl b/internal/command/testdata/test/override_data_nested_list_attribute_partial_elements/main.tftest.hcl
new file mode 100644
index 000000000000..ad5c19f05232
--- /dev/null
+++ b/internal/command/testdata/test/override_data_nested_list_attribute_partial_elements/main.tftest.hcl
@@ -0,0 +1,44 @@
+provider "test" {}
+
+override_data {
+  target = data.test_data_source.datasource
+  values = {
+    nested_list_value = [
+      {
+        name = "first"
+      },
+      {
+        value = "two"
+      },
+    ]
+  }
+}
+
+run "test_override_data_nested_list_attribute_partial_elements" {
+  command = plan
+
+  assert {
+    condition     = length(data.test_data_source.datasource.nested_list_value) == 2
+    error_message = "Expected nested_list_value to have 2 elements, got ${length(data.test_data_source.datasource.nested_list_value)}"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.nested_list_value[0].name == "first"
+    error_message = "Expected first element name to be 'first'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.nested_list_value[0].value != null
+    error_message = "Expected first element value to be filled in"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.nested_list_value[1].value == "two"
+    error_message = "Expected second element value to be 'two'"
+  }
+
+  assert {
+    condition     = data.test_data_source.datasource.nested_list_value[1].name != null
+    error_message = "Expected second element name to be filled in"
+  }
+}
diff --git a/internal/moduletest/mocking/fill.go b/internal/moduletest/mocking/fill.go
index 6eccf33e214d..8d10be61e5c0 100644
--- a/internal/moduletest/mocking/fill.go
+++ b/internal/moduletest/mocking/fill.go
@@ -17,35 +17,50 @@ import (
 // attributes and/or performing conversions to make the input value correct.
 //
 // It is similar to FillType, except it accepts attributes instead of types.
-func FillAttribute(in cty.Value, attribute *configschema.Attribute) (cty.Value, error) {
-	return fillAttribute(in, attribute, cty.Path{})
+func FillAttribute(providedMock cty.Value, attribute *configschema.Attribute) (cty.Value, error) {
+	return fillAttribute(providedMock, attribute, cty.Path{})
 }
 
-func fillAttribute(in cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
+func fillAttribute(providedMock cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
 	ty := attribute.Type
 	if attribute.NestedType != nil {
+		// For nested types, the providedMock value is interpreted in two ways:
+		// - If it's an object, it's treated as a single instance of the nested type,
+		//   and because we can't know how many instances are needed, we return an empty collection.
+		// - If it's a collection, it's treated as the whole nested type collection,
+		// and then we update each element of the collection with generated values where possible.
+		// Note: The collection type must match the attribute's nested type.
 
 		switch attribute.NestedType.Nesting {
 		case configschema.NestingSingle, configschema.NestingGroup:
-			return fillObject(in, attribute, path)
+			return fillObject(providedMock, attribute, path)
 		case configschema.NestingSet:
-			return cty.SetValEmpty(attribute.ImpliedType().ElementType()), nil
+			if providedMock.Type().IsObjectType() {
+				return cty.SetValEmpty(attribute.ImpliedType().ElementType()), nil
+			}
+			return fillIterable(providedMock, attribute, path)
 		case configschema.NestingList:
-			return fillIterable(in, attribute, path)
+			if providedMock.Type().IsObjectType() {
+				return cty.ListValEmpty(attribute.ImpliedType().ElementType()), nil
+			}
+			return fillIterable(providedMock, attribute, path)
 		case configschema.NestingMap:
-			return cty.MapValEmpty(attribute.ImpliedType().ElementType()), nil
+			if providedMock.Type().IsObjectType() {
+				return cty.MapValEmpty(attribute.ImpliedType().ElementType()), nil
+			}
+			return fillIterable(providedMock, attribute, path)
 		default:
 			panic(fmt.Errorf("unknown nesting mode: %d", attribute.NestedType.Nesting))
 		}
 	}
 
-	return fillType(in, ty, path)
+	return fillType(providedMock, ty, path)
 }
 
-func fillObject(in cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
-	// Then the in value must be an object.
-	if !in.Type().IsObjectType() {
-		return cty.NilVal, path.NewErrorf("incompatible types; expected object type, found %s", in.Type().FriendlyName())
+func fillObject(providedMock cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
+	// Then the providedMock value must be an object.
+	if !providedMock.Type().IsObjectType() {
+		return cty.NilVal, path.NewErrorf("incompatible types; expected object type, found %s", providedMock.Type().FriendlyName())
 	}
 
 	var names []string
@@ -63,8 +78,8 @@ func fillObject(in cty.Value, attribute *configschema.Attribute, path cty.Path)
 
 	children := make(map[string]cty.Value)
 	for _, name := range names {
-		if in.Type().HasAttribute(name) {
-			child, err := fillAttribute(in.GetAttr(name), attribute.NestedType.Attributes[name], path.GetAttr(name))
+		if providedMock.Type().HasAttribute(name) {
+			child, err := fillAttribute(providedMock.GetAttr(name), attribute.NestedType.Attributes[name], path.GetAttr(name))
 			if err != nil {
 				return cty.NilVal, err
 			}
@@ -76,9 +91,9 @@ func fillObject(in cty.Value, attribute *configschema.Attribute, path cty.Path)
 	return cty.ObjectVal(children), nil
 }
 
-func fillIterable(in cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
+func fillIterable(providedMock cty.Value, attribute *configschema.Attribute, path cty.Path) (cty.Value, error) {
 	ty := attribute.NestedType.ConfigType()
-	out, err := fillType(in, ty, path)
+	out, err := fillType(providedMock, ty, path)
 	if err != nil {
 		return cty.NilVal, err
 	}

From da295013b552b80778daec9d9948f0a189d7091e Mon Sep 17 00:00:00 2001
From: Samsondeen Dare <samsondeen.dare@hashicorp.com>
Date: Mon, 13 Apr 2026 16:28:43 +0200
Subject: [PATCH 10/11] more tests

---
 internal/command/test_test.go                 |  10 +-
 .../mocking/tests/nested_mocked.tftest.hcl    |   8 +-
 internal/command/testing/test_provider.go     | 100 +++++++++---------
 internal/moduletest/mocking/fill.go           |   5 +-
 internal/moduletest/mocking/values.go         |  15 +++
 5 files changed, 78 insertions(+), 60 deletions(-)

diff --git a/internal/command/test_test.go b/internal/command/test_test.go
index d96840e39aa8..9dabeb4381d5 100644
--- a/internal/command/test_test.go
+++ b/internal/command/test_test.go
@@ -377,12 +377,14 @@ func TestTest_Runs(t *testing.T) {
 			code:        0,
 		},
 		"write-only-attributes-mocked": {
-			expectedOut: []string{"1 passed, 0 failed."},
-			code:        0,
+			expectedOut: []string{"0 passed, 1 failed."},
+			expectedErr: []string{"Test assertion failed", "wrong value"},
+			code:        1,
 		},
 		"write-only-attributes-overridden": {
-			expectedOut: []string{"1 passed, 0 failed."},
-			code:        0,
+			expectedOut: []string{"0 passed, 1 failed."},
+			expectedErr: []string{"Test assertion failed", "wrong value"},
+			code:        1,
 		},
 		"with-default-variables": {
 			args:        []string{"-var=input_two=universe"},
diff --git a/internal/command/testdata/test/mocking/tests/nested_mocked.tftest.hcl b/internal/command/testdata/test/mocking/tests/nested_mocked.tftest.hcl
index 28cb1d9be769..a59ba8e3c350 100644
--- a/internal/command/testdata/test/mocking/tests/nested_mocked.tftest.hcl
+++ b/internal/command/testdata/test/mocking/tests/nested_mocked.tftest.hcl
@@ -8,22 +8,22 @@ mock_provider "test" {
       nested_list_value = [{name  = "first"}, {name  = "second"}]
       set_value = [{name  = "first"}, {name  = "second"}]
       nested_set_value = [{name  = "first"}, {name  = "second"}]
-      map_value = {
+      map_value = tomap({
         "key1": {
           name  = "first"
         },
         "key2": {
           name  = "third"
         }
-      }
-      nested_map_value = {
+      })
+      nested_map_value = tomap({
         "key1": {
           name  = "first"
         },
         "key2": {
           name  = "third"
         }
-      }
+      })
     }
   }
 }
diff --git a/internal/command/testing/test_provider.go b/internal/command/testing/test_provider.go
index a2c3390fb2f9..d60ceab07ce2 100644
--- a/internal/command/testing/test_provider.go
+++ b/internal/command/testing/test_provider.go
@@ -133,69 +133,42 @@ var (
 		DataSources: map[string]providers.Schema{
 			"test_data_source": {
 				Body: &configschema.Block{
-					Attributes: map[string]*configschema.Attribute{
-						"id":         {Type: cty.String, Required: true},
-						"value":      {Type: cty.String, Computed: true},
-						"write_only": {Type: cty.String, Optional: true, WriteOnly: true},
-
-						// list_value is a computed attribute of list-of-objects
-						// type, used to test that override_data can correctly
-						// override list-type attributes (see GitHub issue #37939).
-						"list_value": {
-							Type: cty.List(cty.Object(map[string]cty.Type{
-								"name":  cty.String,
-								"value": cty.String,
-							})),
-							Computed: true,
-						},
-
-						// nested_list_value is a computed attribute using NestedType
-						// with NestingList, used to test that override_data can correctly
-						// override nested list-type attributes (see GitHub issue #37939).
-						// Many real-world providers (e.g. Databricks) define list-of-objects
-						// attributes this way rather than using a plain cty.List type.
-						"nested_list_value": {
-							Computed: true,
-							NestedType: &configschema.Object{
-								Nesting: configschema.NestingList,
-								Attributes: map[string]*configschema.Attribute{
-									"name":  {Type: cty.String, Optional: true},
-									"value": {Type: cty.String, Optional: true},
-								},
-							},
-						},
+					Attributes: withBlockTypeAttributes(map[string]*configschema.Attribute{
+						"id":    {Type: cty.String, Optional: true, Computed: true},
+						"value": {Type: cty.String, Optional: true},
 
 						// We never actually reference these values from a data
 						// source, but we have tests that use the same cty.Value
 						// to represent a test_resource and a test_data_source
 						// so the schemas have to match.
-
-						"interrupt_count":      {Type: cty.Number, Computed: true},
-						"destroy_fail":         {Type: cty.Bool, Computed: true},
-						"create_wait_seconds":  {Type: cty.Number, Computed: true},
-						"destroy_wait_seconds": {Type: cty.Number, Computed: true},
-						"defer":                {Type: cty.Bool, Computed: true},
-					},
+						//
+						"interrupt_count":      {Type: cty.Number, Optional: true},
+						"destroy_fail":         {Type: cty.Bool, Optional: true, Computed: true},
+						"create_wait_seconds":  {Type: cty.Number, Optional: true},
+						"destroy_wait_seconds": {Type: cty.Number, Optional: true},
+						"write_only":           {Type: cty.String, Optional: true, WriteOnly: true},
+						"defer":                {Type: cty.Bool, Optional: true},
+					}),
 				},
 			},
 
 			"test_complex_data_source": {
 				Body: &configschema.Block{
 					Attributes: withBlockTypeAttributes(map[string]*configschema.Attribute{
-						"id":         {Type: cty.String, Required: true},
-						"value":      {Type: cty.String, Computed: true},
-						"write_only": {Type: cty.String, Optional: true, WriteOnly: true},
+						"id":    {Type: cty.String, Optional: true, Computed: true},
+						"value": {Type: cty.String, Optional: true},
 
 						// We never actually reference these values from a data
 						// source, but we have tests that use the same cty.Value
 						// to represent a test_resource and a test_data_source
 						// so the schemas have to match.
-
-						"interrupt_count":      {Type: cty.Number, Computed: true},
-						"destroy_fail":         {Type: cty.Bool, Computed: true},
-						"create_wait_seconds":  {Type: cty.Number, Computed: true},
-						"destroy_wait_seconds": {Type: cty.Number, Computed: true},
-						"defer":                {Type: cty.Bool, Computed: true},
+						//
+						"interrupt_count":      {Type: cty.Number, Optional: true},
+						"destroy_fail":         {Type: cty.Bool, Optional: true, Computed: true},
+						"create_wait_seconds":  {Type: cty.Number, Optional: true},
+						"destroy_wait_seconds": {Type: cty.Number, Optional: true},
+						"write_only":           {Type: cty.String, Optional: true, WriteOnly: true},
+						"defer":                {Type: cty.Bool, Optional: true},
 					}),
 				},
 			},
@@ -498,14 +471,41 @@ func (provider *TestProvider) ReadDataSource(request providers.ReadDataSourceReq
 	resource := provider.Store.Get(provider.GetDataKey(id))
 	if resource == cty.NilVal {
 		diags = diags.Append(tfdiags.Sourceless(tfdiags.Error, "not found", fmt.Sprintf("%s does not exist", id)))
+		resource = cty.EmptyObjectVal
 	}
 
-	if writeOnly := resource.GetAttr("write_only"); !writeOnly.IsNull() {
-		vals := resource.AsValueMap()
+	vals := resource.AsValueMap()
+
+	if writeOnly := request.Config.GetAttr("write_only"); !writeOnly.IsNull() {
 		vals["write_only"] = cty.NullVal(cty.String)
-		resource = cty.ObjectVal(vals)
 	}
 
+	sharedObjectType := cty.Object(map[string]cty.Type{
+		"name":  cty.String,
+		"value": cty.String,
+	})
+
+	if _, exists := vals["list_value"]; !exists {
+		vals["list_value"] = cty.ListValEmpty(sharedObjectType)
+	}
+	if _, exists := vals["set_value"]; !exists {
+		vals["set_value"] = cty.SetValEmpty(sharedObjectType)
+	}
+	if _, exists := vals["map_value"]; !exists {
+		vals["map_value"] = cty.MapValEmpty(sharedObjectType)
+	}
+	if _, exists := vals["nested_list_value"]; !exists {
+		vals["nested_list_value"] = cty.ListValEmpty(sharedObjectType)
+	}
+	if _, exists := vals["nested_set_value"]; !exists {
+		vals["nested_set_value"] = cty.SetValEmpty(sharedObjectType)
+	}
+	if _, exists := vals["nested_map_value"]; !exists {
+		vals["nested_map_value"] = cty.MapValEmpty(sharedObjectType)
+	}
+
+	resource = cty.ObjectVal(vals)
+
 	return providers.ReadDataSourceResponse{
 		State:       resource,
 		Diagnostics: diags,
diff --git a/internal/moduletest/mocking/fill.go b/internal/moduletest/mocking/fill.go
index 8d10be61e5c0..1c468fe2680c 100644
--- a/internal/moduletest/mocking/fill.go
+++ b/internal/moduletest/mocking/fill.go
@@ -27,8 +27,9 @@ func fillAttribute(providedMock cty.Value, attribute *configschema.Attribute, pa
 		// For nested types, the providedMock value is interpreted in two ways:
 		// - If it's an object, it's treated as a single instance of the nested type,
 		//   and because we can't know how many instances are needed, we return an empty collection.
-		// - If it's a collection, it's treated as the whole nested type collection,
-		// and then we update each element of the collection with generated values where possible.
+		// - If it's already a collection, it's treated as the whole nested type
+		//   collection, and then we update each element of the collection with
+		//   generated values where possible.
 		// Note: The collection type must match the attribute's nested type.
 
 		switch attribute.NestedType.Nesting {
diff --git a/internal/moduletest/mocking/values.go b/internal/moduletest/mocking/values.go
index 6048aee35888..2b8b77618667 100644
--- a/internal/moduletest/mocking/values.go
+++ b/internal/moduletest/mocking/values.go
@@ -165,6 +165,21 @@ func populateComputedValues(target cty.Value, with MockedData, schema *configsch
 		})
 	}
 
+	// If there are no errors, we coerce the value to ensure it matches the schema.
+	// Any errors here would be because we generated an invalid value.
+	if !diags.HasErrors() {
+		var err error
+		value, err = schema.CoerceValue(value)
+		if err != nil {
+			diags = diags.Append(&hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Detail:   "Failed to coerce value",
+				Summary:  fmt.Sprintf("Terraform failed to coerce a value for a mocked object: %s. This is a bug in Terraform - please report it.", err),
+				Subject:  with.Range.Ptr(),
+			})
+		}
+	}
+
 	return value, diags
 }
 

From fc4e2d595f41c6809f36e892d3b1be3f7737063f Mon Sep 17 00:00:00 2001
From: Samsondeen Dare <samsondeen.dare@hashicorp.com>
Date: Mon, 13 Apr 2026 16:47:45 +0200
Subject: [PATCH 11/11] update tests

---
 internal/moduletest/mocking/values_test.go | 53 +++++++++++++++++-----
 1 file changed, 41 insertions(+), 12 deletions(-)

diff --git a/internal/moduletest/mocking/values_test.go b/internal/moduletest/mocking/values_test.go
index 1757be11cc51..a75ff15302a1 100644
--- a/internal/moduletest/mocking/values_test.go
+++ b/internal/moduletest/mocking/values_test.go
@@ -570,7 +570,7 @@ func TestComputedValuesForDataSource(t *testing.T) {
 				}),
 			}),
 		},
-		"nested_list_attribute_computed_err": {
+		"nested_list_attribute_computed_empty": {
 			target: cty.ObjectVal(map[string]cty.Value{
 				"nested": cty.NullVal(cty.List(cty.Object(map[string]cty.Type{
 					"id":    cty.String,
@@ -599,9 +599,7 @@ func TestComputedValuesForDataSource(t *testing.T) {
 					"value": cty.String,
 				})),
 			}),
-			expectedFailures: []string{
-				`Terraform could not compute a value for the target type list of object with the mocked data defined at :0,0-0 with the attribute ".nested": incompatible types; expected list of object, found object.`,
-			},
+			expectedFailures: nil,
 		},
 		"nested_set_attribute": {
 			target: cty.ObjectVal(map[string]cty.Value{
@@ -704,7 +702,7 @@ func TestComputedValuesForDataSource(t *testing.T) {
 				}),
 			}),
 		},
-		"nested_set_attribute_computed_err": {
+		"nested_set_attribute_computed_empty": {
 			target: cty.ObjectVal(map[string]cty.Value{
 				"nested": cty.NullVal(cty.Set(cty.Object(map[string]cty.Type{
 					"id":    cty.String,
@@ -733,9 +731,7 @@ func TestComputedValuesForDataSource(t *testing.T) {
 					"value": cty.String,
 				})),
 			}),
-			expectedFailures: []string{
-				`Terraform could not compute a value for the target type set of object with the mocked data defined at :0,0-0 with the attribute ".nested": incompatible types; expected set of object, found object.`,
-			},
+			expectedFailures: nil,
 		},
 		"nested_map_attribute": {
 			target: cty.ObjectVal(map[string]cty.Value{
@@ -838,7 +834,7 @@ func TestComputedValuesForDataSource(t *testing.T) {
 				}),
 			}),
 		},
-		"nested_map_attribute_computed_err": {
+		"nested_map_attribute_computed_empty": {
 			target: cty.ObjectVal(map[string]cty.Value{
 				"nested": cty.NullVal(cty.Map(cty.Object(map[string]cty.Type{
 					"id":    cty.String,
@@ -867,9 +863,7 @@ func TestComputedValuesForDataSource(t *testing.T) {
 					"value": cty.String,
 				})),
 			}),
-			expectedFailures: []string{
-				`Terraform could not compute a value for the target type map of object with the mocked data defined at :0,0-0 with the attribute ".nested[\"id\"]": incompatible types; expected object, found string.`,
-			},
+			expectedFailures: nil,
 		},
 		"invalid_replacement_path": {
 			target: cty.ObjectVal(map[string]cty.Value{
@@ -1047,6 +1041,41 @@ func TestComputedValuesForDataSource(t *testing.T) {
 				"Terraform could not compute a value for the target type string with the mocked data defined at :0,0-0 with the attribute \".block[0].id\": string required, but have object.",
 			},
 		},
+
+		"invalid_replacement_path_nested_list_block": {
+			target: cty.ObjectVal(map[string]cty.Value{
+				"block": cty.ListVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"id":    cty.NullVal(cty.String),
+						"value": cty.StringVal("one"),
+					}),
+				}),
+			}),
+			with: cty.ObjectVal(map[string]cty.Value{
+				"block": cty.ListVal([]cty.Value{
+					cty.StringVal("Hello, world!"),
+				}),
+			}),
+			schema: &configschema.Block{
+				BlockTypes: map[string]*configschema.NestedBlock{
+					"block": {
+						Block:   computedBlock,
+						Nesting: configschema.NestingList,
+					},
+				},
+			},
+			expected: cty.ObjectVal(map[string]cty.Value{
+				"block": cty.ListVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"id":    cty.StringVal("ssnk9qhr"),
+						"value": cty.StringVal("one"),
+					}),
+				}),
+			}),
+			expectedFailures: []string{
+				"Terraform expected an object type for attribute \".block[0]\" defined within the mocked data at :0,0-0, but found list of string.",
+			},
+		},
 		"dynamic_attribute_unset": {
 			target: cty.ObjectVal(map[string]cty.Value{
 				"dynamic_attribute": cty.NullVal(cty.DynamicPseudoType),