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

[thomhurst]

Summary

Fixes #6516 — T.Mock() on an abstract class with an abstract indexer failed to compile with CS0205: Cannot call an abstract base member, because the generated override always emitted a return base[...]; / base[...] = value; fallback.

Root cause

CreateIndexerModel in MemberDiscovery never set IsAbstractMember / IsVirtualMember, so every class indexer — abstract or not — took the virtual-member path in GenerateOverrideIndexer, whose fallback calls base[...]. Abstract indexers have no base implementation, so the mock impl didn't compile. Abstract methods and non-indexer properties already carried the flag and dispatched engine-only; indexers were the one member kind that didn't.

Fix

Set IsAbstractMember = indexer.IsAbstract (and IsVirtualMember for parity with the method/property models) on the indexer model. The existing abstract branches in GenerateOverrideIndexer then emit engine-only dispatch — unconfigured calls return the smart default (or throw in strict mode), exactly like other abstract members. Virtual indexers keep their base fallback (covered by a regression test).

This unblocks mocking System.Data.Common.DbDataReader (two abstract indexers), reported as blocking an NSubstitute migration.

Tests

• Issue6516Tests (runtime): issue repro shape (two abstract get-only indexers, int + string keys), abstract get/set indexer setter dispatch, virtual indexer base-fallback regression. 4/4 pass; full TUnit.Mocks.Tests suite 1186/1186 on net10.0.
• Abstract_Class_With_Abstract_Indexer (source generator): asserts no base[...] in abstract indexer bodies, base fallback retained for the virtual one, no CS0205 in the generated compilation + snapshot. Full generator suite 91/91.

[greptile-apps]

Greptile Summary

Fixes abstract-indexer mock generation by classifying indexers consistently with methods and properties.

• Abstract indexers now dispatch directly through the mock engine instead of generating invalid abstract base calls.
• Virtual and override indexers retain their existing base-fallback behavior.
• Adds source-generator snapshots and runtime coverage for get-only, read/write, configured, unconfigured, and virtual indexers.

Confidence Score: 5/5

The PR appears safe to merge with no actionable defects identified.

The indexer flags now match the established method and property modeling behavior, and the added tests cover both corrected abstract dispatch and preserved virtual fallback behavior.

Important Files Changed

Filename	Overview
src/TUnit.Mocks.SourceGenerator/Discovery/MemberDiscovery.cs	Adds abstract and virtual classification to indexer models, matching the established method and property classification.
tests/TUnit.Mocks.SourceGenerator.Tests/MockGeneratorTests.cs	Adds generated-compilation and output assertions covering abstract and virtual indexer dispatch.
tests/TUnit.Mocks.SourceGenerator.Tests/Snapshots/Abstract_Class_With_Abstract_Indexer.verified.txt	Captures engine-only abstract indexer implementations and retained virtual base fallbacks.
tests/TUnit.Mocks.Tests/Issue6516Tests.cs	Adds runtime regressions for abstract getter/setter dispatch, setup and verification, smart defaults, and virtual fallback behavior.

_{Reviews (1): Last reviewed commit: "fix(mocks): dispatch abstract indexers t..." | Re-trigger Greptile}

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

The fix is minimal and correctly scoped: it sets IsAbstractMember/IsVirtualMember on the indexer model in CreateIndexerModel, mirroring the exact same flags already set for methods (MemberDiscovery.cs:746-747) and non-indexer properties (MemberDiscovery.cs:815-816). GenerateOverrideIndexer already branches on IsAbstractMember to dispatch engine-only vs. base-fallback, so no codegen changes were needed — just correct classification.

• Consistent with established patterns for methods/properties; no new abstraction or edge case introduced.
• Well tested: source-gen snapshot test asserts no base[...] calls in abstract indexer bodies and retains base fallback for the virtual case, plus runtime regression tests covering get-only, read/write, and virtual indexers.
• Snapshot file was added correctly (.verified.txt, no stray .received.txt).
• Root CLAUDE.md is the only one in scope for the changed paths; no violations found.