fix(mocks): dispatch abstract indexers through the engine instead of calling base

src/TUnit.Mocks.SourceGenerator/Discovery/MemberDiscovery.cs

@@ -915,6 +915,8 @@ private static MockMemberModel CreateIndexerModel(IPropertySymbol indexer, ref i
             DeclaringInterfaceName = declaringInterfaceName,
             NullableAnnotation = indexer.Type.NullableAnnotation.ToString(),
             SmartDefault = indexer.Type.GetSmartDefault(indexer.Type.IsNullableAnnotated()),
+            IsAbstractMember = indexer.IsAbstract,
+            IsVirtualMember = indexer.IsVirtual || indexer.IsOverride,
             OverrideAccessModifier = overrideAccessModifier,
             GetterAccessModifier = GetAccessorAccessModifier(indexer.GetMethod, overrideAccessModifier, compilationAssembly),
             SetterAccessModifier = GetAccessorAccessModifier(indexer.SetMethod, overrideAccessModifier, compilationAssembly),

tests/TUnit.Mocks.SourceGenerator.Tests/MockGeneratorTests.cs

@@ -2382,6 +2382,45 @@ public class TestUsage
         AssertNoGeneratedError(source, "CS0535");
     }
 
+    [Test]
+    public Task Abstract_Class_With_Abstract_Indexer()
+    {
+        // Regression #6516: an abstract indexer took the virtual-member path and emitted a
+        // `base[...]` fallback — CS0205, there is no base implementation. Abstract indexers must
+        // dispatch through the engine only, like abstract methods/properties (DbDataReader shape).
+        var source = """
+            using TUnit.Mocks;
+
+            public abstract class Repository
+            {
+                public abstract object this[int index] { get; }
+                public abstract object this[string key] { get; }
+                public abstract string this[int a, int b] { get; set; }
+                public virtual string this[bool flag] { get => "base"; set { } }
+            }
+
+            public class TestUsage
+            {
+                void M()
+                {
+                    var mock = Repository.Mock();
+                }
+            }
+            """;
+
+        var output = GetGeneratedOutput(source);
+
+        // Virtual indexer keeps its base fallback; abstract ones must not reference base at all.
+        AssertContains(output, "return base[flag];");
+        AssertDoesNotContain(output, "return base[index];");
+        AssertDoesNotContain(output, "return base[key];");
+        AssertDoesNotContain(output, "base[a, b]");
+
+        AssertNoGeneratedError(source, "CS0205");
+
+        return VerifyGeneratorOutput(source);
+    }
+
     private static void AssertNoGeneratedError(string source, string errorId)
     {
         foreach (var diagnostic in GetGeneratedCompilationErrors(source))

tests/TUnit.Mocks.SourceGenerator.Tests/Snapshots/Abstract_Class_With_Abstract_Indexer.verified.txt

@@ -0,0 +1,203 @@
+// <auto-generated/>
+#pragma warning disable
+#nullable enable
+
+file sealed class RepositoryMockImpl : global::Repository, global::TUnit.Mocks.IRaisable, global::TUnit.Mocks.IMockObject
+{
+    private readonly global::TUnit.Mocks.MockEngine<global::Repository> _engine;
+
+    [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+    global::TUnit.Mocks.IMock? global::TUnit.Mocks.IMockObject.MockWrapper { get; set; }
+
+    [global::System.Diagnostics.CodeAnalysis.SetsRequiredMembers]
+    internal RepositoryMockImpl(global::TUnit.Mocks.MockEngine<global::Repository> engine) : base()
+    {
+        _engine = engine;
+    }
+
+    public override object this[int index]
+    {
+        get => _engine.HandleCallWithReturn<object>(0, "get_Item", new object?[] { index }, default!);
+    }
+
+    public override object this[string key]
+    {
+        get => _engine.HandleCallWithReturn<object>(1, "get_Item", new object?[] { key }, default!);
+    }
+
+    public override string this[int a, int b]
+    {
+        get => _engine.HandleCallWithReturn<string>(2, "get_Item", new object?[] { a, b }, "");
+        set => _engine.HandleCall(3, "set_Item", new object?[] { a, b, value });
+    }
+
+    public override string this[bool flag]
+    {
+        get
+        {
+            if (_engine.TryHandleCallWithReturn<string>(4, "get_Item", new object?[] { flag }, "", out var __result))
+            {
+                return __result;
+            }
+            return base[flag];
+        }
+        set
+        {
+            if (!_engine.TryHandleCall(5, "set_Item", new object?[] { flag, value }))
+            {
+                base[flag] = value;
+            }
+        }
+    }
+
+    [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+    public void RaiseEvent(string eventName, object? args)
+    {
+        throw new global::System.InvalidOperationException($"No event named '{eventName}' exists on this mock.");
+    }
+}
+
+file static class RepositoryPartialMockFactory
+{
+    [global::System.Runtime.CompilerServices.ModuleInitializer]
+    internal static void Register()
+    {
+        global::TUnit.Mocks.MockRegistry.RegisterFactory<global::Repository>(Create);
+    }
+
+    private static global::TUnit.Mocks.Mock<global::Repository> Create(global::TUnit.Mocks.MockBehavior behavior, object[] constructorArgs)
+    {
+        var engine = new global::TUnit.Mocks.MockEngine<global::Repository>(behavior);
+        var impl = new RepositoryMockImpl(engine);
+        engine.Raisable = impl;
+        var mock = new global::TUnit.Mocks.Mock<global::Repository>(impl, engine);
+        return mock;
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#pragma warning disable
+#nullable enable
+
+namespace TUnit.Mocks.Generated
+{
+    public static class Repository_MockMemberExtensions
+    {
+        public static global::TUnit.Mocks.MockMethodCall<object> Item(this global::TUnit.Mocks.Mock<global::Repository> mock, global::TUnit.Mocks.Arguments.Arg<int> index)
+        {
+            var matchers = new global::TUnit.Mocks.Arguments.IArgumentMatcher[] { index.Matcher };
+            return new global::TUnit.Mocks.MockMethodCall<object>(global::TUnit.Mocks.MockRegistry.GetEngine(mock), 0, "get_Item", matchers);
+        }
+
+        public static global::TUnit.Mocks.MockMethodCall<object> Item(this global::TUnit.Mocks.Mock<global::Repository> mock, global::TUnit.Mocks.Arguments.Arg<string> key)
+        {
+            var matchers = new global::TUnit.Mocks.Arguments.IArgumentMatcher[] { key.Matcher };
+            return new global::TUnit.Mocks.MockMethodCall<object>(global::TUnit.Mocks.MockRegistry.GetEngine(mock), 1, "get_Item", matchers);
+        }
+
+        public static global::TUnit.Mocks.MockMethodCall<string> Item(this global::TUnit.Mocks.Mock<global::Repository> mock, global::TUnit.Mocks.Arguments.Arg<int> a, global::TUnit.Mocks.Arguments.Arg<int> b)
+        {
+            var matchers = new global::TUnit.Mocks.Arguments.IArgumentMatcher[] { a.Matcher, b.Matcher };
+            return new global::TUnit.Mocks.MockMethodCall<string>(global::TUnit.Mocks.MockRegistry.GetEngine(mock), 2, "get_Item", matchers);
+        }
+
+        public static global::TUnit.Mocks.VoidMockMethodCall SetItem(this global::TUnit.Mocks.Mock<global::Repository> mock, global::TUnit.Mocks.Arguments.Arg<int> a, global::TUnit.Mocks.Arguments.Arg<int> b, global::TUnit.Mocks.Arguments.Arg<string> value)
+        {
+            var matchers = new global::TUnit.Mocks.Arguments.IArgumentMatcher[] { a.Matcher, b.Matcher, value.Matcher };
+            return new global::TUnit.Mocks.VoidMockMethodCall(global::TUnit.Mocks.MockRegistry.GetEngine(mock), 3, "set_Item", matchers);
+        }
+
+        public static global::TUnit.Mocks.MockMethodCall<string> Item(this global::TUnit.Mocks.Mock<global::Repository> mock, global::TUnit.Mocks.Arguments.Arg<bool> flag)
+        {
+            var matchers = new global::TUnit.Mocks.Arguments.IArgumentMatcher[] { flag.Matcher };
+            return new global::TUnit.Mocks.MockMethodCall<string>(global::TUnit.Mocks.MockRegistry.GetEngine(mock), 4, "get_Item", matchers);
+        }
+
+        public static global::TUnit.Mocks.VoidMockMethodCall SetItem(this global::TUnit.Mocks.Mock<global::Repository> mock, global::TUnit.Mocks.Arguments.Arg<bool> flag, global::TUnit.Mocks.Arguments.Arg<string> value)
+        {
+            var matchers = new global::TUnit.Mocks.Arguments.IArgumentMatcher[] { flag.Matcher, value.Matcher };
+            return new global::TUnit.Mocks.VoidMockMethodCall(global::TUnit.Mocks.MockRegistry.GetEngine(mock), 5, "set_Item", matchers);
+        }
+
+        #if NET9_0_OR_GREATER
+        [global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
+        public static void Reset(this global::TUnit.Mocks.Mock<global::Repository> mock)
+            => global::TUnit.Mocks.Mock.Reset(mock);
+
+        [global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
+        public static void VerifyAll(this global::TUnit.Mocks.Mock<global::Repository> mock)
+            => global::TUnit.Mocks.Mock.VerifyAll(mock);
+
+        [global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
+        public static void VerifyNoOtherCalls(this global::TUnit.Mocks.Mock<global::Repository> mock)
+            => global::TUnit.Mocks.Mock.VerifyNoOtherCalls(mock);
+
+        [global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
+        public static void SetupAllProperties(this global::TUnit.Mocks.Mock<global::Repository> mock)
+            => global::TUnit.Mocks.Mock.SetupAllProperties(mock);
+
+        [global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
+        public static global::TUnit.Mocks.Diagnostics.MockDiagnostics GetDiagnostics(this global::TUnit.Mocks.Mock<global::Repository> mock)
+            => global::TUnit.Mocks.Mock.GetDiagnostics(mock);
+
+        [global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
+        public static void SetState(this global::TUnit.Mocks.Mock<global::Repository> mock, string? stateName)
+            => global::TUnit.Mocks.Mock.SetState(mock, stateName);
+
+        [global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
+        public static void InState(this global::TUnit.Mocks.Mock<global::Repository> mock, string stateName, global::System.Action<global::TUnit.Mocks.Mock<global::Repository>> configure)
+            => global::TUnit.Mocks.Mock.InState(mock, stateName, configure);
+
+        extension(global::TUnit.Mocks.Mock<global::Repository> mock)
+        {
+            public global::System.Collections.Generic.IReadOnlyList<global::TUnit.Mocks.Verification.CallRecord> Invocations => global::TUnit.Mocks.Mock.Invocations(mock);
+
+            public global::TUnit.Mocks.MockBehavior Behavior => global::TUnit.Mocks.Mock.Behavior(mock);
+
+            public global::TUnit.Mocks.IDefaultValueProvider? DefaultValueProvider
+            {
+                get => global::TUnit.Mocks.Mock.GetDefaultValueProvider(mock);
+                set => global::TUnit.Mocks.Mock.SetDefaultValueProvider(mock, value);
+            }
+        }
+        #endif
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#pragma warning disable
+#nullable enable
+
+namespace TUnit.Mocks
+{
+    public static class Repository_MockStaticExtension
+    {
+        extension(global::Repository _)
+        {
+            public static global::TUnit.Mocks.Mock<global::Repository> Mock()
+            {
+                return global::TUnit.Mocks.Mock.Of<global::Repository>();
+            }
+
+            public static global::TUnit.Mocks.Mock<global::Repository> Mock(global::TUnit.Mocks.MockBehavior behavior)
+            {
+                return global::TUnit.Mocks.Mock.Of<global::Repository>(behavior);
+            }
+        }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#pragma warning disable
+#nullable enable
+
+namespace TUnit.Mocks.Generated;

tests/TUnit.Mocks.Tests/Issue6516Tests.cs

@@ -0,0 +1,89 @@
+using TUnit.Mocks;
+using TUnit.Mocks.Arguments;
+using TUnit.Mocks.Verification;
+
+namespace TUnit.Mocks.Tests;
+
+// Regression: https://github.com/thomhurst/TUnit/issues/6516
+// The indexer model never carried IsAbstractMember, so an abstract indexer on a mocked
+// abstract class took the virtual-member path and emitted a `return base[...];` /
+// `base[...] = value;` fallback — CS0205, there is no base implementation to call.
+// Abstract indexers must dispatch through the engine only, exactly like abstract
+// methods and abstract non-indexer properties already do.
+
+#region Test types
+
+// The issue's repro shape (mirrors DbDataReader's `this[int]` / `this[string]`).
+public abstract class AbstractIndexerRepository
+{
+    public abstract object this[int index] { get; }
+    public abstract object this[string key] { get; }
+}
+
+// Exercises the abstract setter branch too.
+public abstract class AbstractReadWriteIndexer
+{
+    public abstract string this[int index] { get; set; }
+}
+
+// Virtual indexer must keep its base fallback.
+public class VirtualIndexerStore
+{
+    public virtual string this[int index]
+    {
+        get => $"base-{index}";
+        set { }
+    }
+}
+
+#endregion
+
+public class Issue6516Tests
+{
+    [Test]
+    public async Task Abstract_GetOnly_Indexers_Compile_And_Return_Default_When_Unconfigured()
+    {
+        var mock = AbstractIndexerRepository.Mock();
+
+        await Assert.That(mock.Object[0]).IsNull();
+        await Assert.That(mock.Object["missing"]).IsNull();
+    }
+
+    [Test]
+    public async Task Abstract_Indexer_Configured_Via_Setup_Returns_Value()
+    {
+        var mock = AbstractIndexerRepository.Mock();
+        mock.Item(3).Returns("three");
+        mock.Item("name").Returns("by-key");
+
+        await Assert.That(mock.Object[3]).IsEqualTo("three");
+        await Assert.That(mock.Object["name"]).IsEqualTo("by-key");
+
+        mock.Item(3).WasCalled(Times.Once);
+        mock.Item("name").WasCalled(Times.Once);
+    }
+
+    [Test]
+    public async Task Abstract_ReadWrite_Indexer_Setter_Dispatches_Through_Engine()
+    {
+        var mock = AbstractReadWriteIndexer.Mock();
+        mock.Item(1).Returns("one");
+
+        await Assert.That(mock.Object[1]).IsEqualTo("one");
+
+        mock.Object[7] = "seven";
+
+        mock.SetItem(7, "seven").WasCalled(Times.Once);
+        mock.SetItem(Any<int>(), Any<string>()).WasCalled(Times.Once);
+    }
+
+    [Test]
+    public async Task Virtual_Indexer_Still_Falls_Back_To_Base_When_Unconfigured()
+    {
+        var mock = VirtualIndexerStore.Mock();
+        mock.Item(1).Returns("configured");
+
+        await Assert.That(mock.Object[1]).IsEqualTo("configured");
+        await Assert.That(mock.Object[2]).IsEqualTo("base-2");
+    }
+}