fix(mocks): emit valid lambdas for Task/ValueTask-returning delegate mocks

[thomhurst]

Fixes #6887

Problem

Mock.OfDelegate<Func<int, Task>>() failed to compile with CS1643. A non-generic Task/ValueTask return is modelled with IsVoid = true, so MockDelegateFactoryBuilder took the Action branch and emitted a lambda without a return.

Task<T>/ValueTask<T> delegates compiled, but dispatched through HandleCallWithReturn<Task<T>> while the shared setup surface is keyed on the unwrapped T. So Returns(value) and ReturnsAsync(...) did not behave as they do for interface members.

Fix

The delegate factory now mirrors the interface-member async path in MockImplBuilder:

• Task/ValueTask: dispatch via HandleCall, honour ReturnsAsync, return a completed task.
• Task<T>/ValueTask<T>: dispatch on the unwrapped T with its smart default, wrap the result, honour ReturnsAsync.
• Exceptions from the engine (e.g. Throws<T>(), strict mode) are returned as faulted tasks.

EmitRawReturnCheck is now internal so both builders share it.

Tests

• DelegateMockTests: runtime coverage for Func<int, Task>, Func<string, ValueTask>, Func<int, Task<int>>, Func<int, Task<string>>, Func<int, ValueTask<int>>, plus Throws and ReturnsAsync.
• Issue6887Tests: generator snapshot for the four async delegate shapes.
• TUnit.Mocks.Tests (1329) and TUnit.Mocks.SourceGenerator.Tests (161) pass on net10.0.

Summary by CodeRabbit

• New Features
  • Delegate mocks support Task, Task<T>, ValueTask, and ValueTask<T> return types, including configured results, default values, and exceptions.
  • Delegate mocks can configure and return values through out and ref parameters, including for asynchronous delegates.
• Bug Fixes
  • Async delegate calls preserve pending results until completion and handle parameter names that could conflict with generated code.
  • User-defined types named Task<T> or ValueTask<T> are treated as regular return types.

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-27T10:46:03.738894Z	68790bd	New commits

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📒 Files selected for processing (3)

• src/TUnit.Mocks.SourceGenerator/Builders/MockDelegateFactoryBuilder.cs
• src/TUnit.Mocks.SourceGenerator/Builders/MockImplBuilder.cs
• tests/TUnit.Mocks.Tests/DelegateMockTests.cs

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📝 Walkthrough

Walkthrough

The generator now emits delegate bodies for Task and ValueTask returns and copies configured out/ref values back to delegate parameters. It uses positional lambda parameter names and recognizes framework async types by namespace. Tests cover generated code and runtime behavior.

Changes

Delegate mock generation

Layer / File(s)	Summary
Identify framework async return types src/TUnit.Mocks.SourceGenerator/Extensions/TypeSymbolExtensions.cs, tests/TUnit.Mocks.SourceGenerator.Tests/Issue6887Tests.cs	Generic Task and ValueTask types are detected and unwrapped only when they have one type argument and belong to System.Threading.Tasks. Tests check user-defined types with the same names.
Generate delegate calls and copy out/ref values src/TUnit.Mocks.SourceGenerator/Builders/MockDelegateFactoryBuilder.cs, src/TUnit.Mocks.SourceGenerator/Builders/MockImplBuilder.cs	Generated lambdas use positional parameter names. Setup-matching arguments omit out parameters. Delegate calls copy configured out/ref values back to parameters. The readback helper accepts an optional parameter-name mapper.
Validate generated and runtime delegates tests/TUnit.Mocks.SourceGenerator.Tests/Issue6887Tests.cs, tests/TUnit.Mocks.SourceGenerator.Tests/Snapshots/Delegates_Returning_Task_And_ValueTask.verified.txt, tests/TUnit.Mocks.Tests/DelegateMockTests.cs	Tests cover generated async delegate code, parameter-name collisions, user-defined Task and ValueTask types, async results and exceptions, task identity, pending results, and configured out/ref values.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 68790

No remaining issue is established that would prevent merging after normal checks.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 68790

The changes are confined to mock generation and its existing runtime dispatch path. No new privileged or externally reachable security path was established, but some state-handoff edge cases remain incompletely covered.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The evidenced behavioral reach is consumers of generated delegate mocks and their configured setups, rather than a newly identified service, tenant, credential, or datastore boundary.

Trust Boundaries and Controls

• observed — Setup values enter through index-keyed assignments. MockEngine replaces the thread-local handoff for a matched or unmatched call, and readback clears it on consumption; these are state-ownership controls, not an authorization boundary.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 36.36% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 33 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: fixing generated lambdas for delegates that return Task or ValueTask.
Linked Issues check	✅ Passed	Issue #6887 requires Mock.OfDelegate to compile for Func<T, Task>. The delegate builder now emits a returned async result for non-generic Task delegates. The snapshot covers Func<int, Task>, a…
Out of Scope Changes check	✅ Passed	The changes remain within the async delegate mock fix for issue #6887. Support for Task<T>, ValueTask, ValueTask<T>, framework type checks, generated parameter names, out/ref readback, snapshots…

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Task and ValueTask now return
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[greptile-apps]

Confidence Score: 5/5

[Medium risk] Fixes code generation for async delegate mocks.

The PR appears safe to merge; no outstanding findings or new actionable issues remain.

Summary

The PR fixes Task- and ValueTask-returning delegate mocks by dispatching generic results through their unwrapped types and preserving configured async returns. The latest changes also apply configured out/ref values to delegate arguments and add runtime coverage for those paths.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[Mocked delegate invocation] --> B[Dispatch to mock engine]
  B --> C[Read back configured out/ref values]
  C --> D{Async return?}
  D -->|Yes| E[Return configured task or wrap result]
  D -->|No| F[Return synchronous result]

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Actionable comments posted: 1

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Inline comments:
In @src/TUnit.Mocks.SourceGenerator/Builders/MockDelegateFactoryBuilder.cs:
- Line 105: Update MockDelegateFactoryBuilder’s generated async delegate code to
allocate collision-free names for __result, __ex, __rawAsync, and __typedAsync,
ensuring none match any parameter name in method.Parameters.

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Reviewing files that changed from the base of the PR and between 0ec4543 and 9f77125.

📒 Files selected for processing (5)

• src/TUnit.Mocks.SourceGenerator/Builders/MockDelegateFactoryBuilder.cs
• src/TUnit.Mocks.SourceGenerator/Builders/MockImplBuilder.cs
• tests/TUnit.Mocks.SourceGenerator.Tests/Issue6887Tests.cs
• tests/TUnit.Mocks.SourceGenerator.Tests/Snapshots/Delegates_Returning_Task_And_ValueTask.verified.txt
• tests/TUnit.Mocks.Tests/DelegateMockTests.cs

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[github-actions]

Review: fix(mocks): emit valid lambdas for Task/ValueTask-returning delegate mocks

Verdict: Looks good — approving.

The fix is correct and well-scoped

MockDelegateFactoryBuilder.Build previously branched only on invokeMethod.IsVoid, which is also true for non-generic Task/ValueTask returns. That routed async delegates into the Action-style branch, emitting a lambda body with no return statement (CS1643) — exactly #6887. The new !invokeMethod.IsAsync guard plus the dedicated EmitAsyncBody path fixes this correctly, and mirrors the try/catch → RawReturnContext check → wrap-or-fault pattern already used in MockImplBuilder.GenerateEngineDispatchBody (now exposed via the newly-internal EmitRawReturnCheck) rather than duplicating it — the right level of reuse per this repo's shared-execution-path convention.

Both new test files exercise the fixed paths well: the generator snapshot (Issue6887Tests) covers Func<int,Task>, Func<int,Task<int>>, Func<ValueTask>, Func<string,ValueTask<string>>, and the runtime tests (DelegateMockTests) cover default-completion, configured results, Throws, and ReturnsAsync with a preserved Task instance across both Task and ValueTask shapes.

Checked against the automated bot findings on this PR

The greptile-apps review flagged two P1s ("delegate parameter names collide" and "custom task types fail compilation"). I couldn't fetch the exact comment bodies (gh api / curl / WebFetch were all sandboxed in this run), so I verified independently against the source:

• Parameter-name collision: real, but pre-existing and out of scope. The lambda body calls engine.HandleCall(...)/engine.HandleCallWithReturn(...) against a captured local named engine; a delegate parameter literally named engine (or behavior, mock, del, constructorArgs) would shadow it. This exists identically in the old code (both the void and returning branches already called engine.Handle*) — this PR doesn't introduce or worsen it.
• Custom task types fail compilation: also real, but traced to IsAsyncReturnType/GetUnwrappedReturnType in TypeSymbolExtensions.cs (untouched by this PR), which match generic Task<T>/ValueTask<T> by simple name only (named.ConstructedFrom.Name == "Task"), not full namespace. A user type literally named Task<T> outside System.Threading.Tasks would be misclassified as async. This affects interface-method mocks the same way today (same shared detection + the pattern this PR mirrors), so it's a pre-existing gap in the type classifier, not something introduced here. For delegates specifically, this PR does change that one obscure case from "compiles but silently wrong" (old code fell through to the synchronous HandleCallWithReturn<TReturn> branch) to "fails to compile" — arguably a wash, and not worth blocking this bug fix on.

Neither is a regression caused by this change; both would be reasonable follow-ups against the shared type-classification helpers rather than against MockDelegateFactoryBuilder.

Minor, non-blocking

• Test coverage for the raw-ReturnsAsync-type-mismatch throw path (RawReturnContext.TryConsume returning a value of the wrong type) and a ValueTask ReturnsAsync case aren't covered by the new tests, per the greptile P2 note. Not required for this fix, but would round things out.

No changes requested.

[github-actions]

Code review

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

Summary: Fixes Mock.OfDelegate<Func<int, Task>>() failing to compile (CS1643) — a non-generic Task/ValueTask return sets IsVoid = true, so the delegate factory took the Action branch and emitted a lambda with no return. The fix reorders the branch checks (IsAsync before the sync IsVoid check) and adds an EmitAsyncBody helper that mirrors the existing interface-member async dispatch pattern in MockImplBuilder.GenerateEngineDispatchBody (try/catch wrapping HandleCall/HandleCallWithReturn, honoring ReturnsAsync via EmitRawReturnCheck, faulting the task on exception) — good reuse of established conventions rather than a divergent implementation.

Two nice secondary fixes bundled in:

• Lambda parameters are now emitted with generator-owned positional names (arg0, arg1, ...) instead of the user's parameter names, avoiding a real CS0136 collision when a delegate parameter is named __result, __ex, engine, del, or mock (covered by Delegate_Parameter_Names_Do_Not_Collide_With_Generated_Locals).
• IsFrameworkGenericTask now checks the containing namespace in addition to the simple name, so a user-defined Task<T>/ValueTask<T> type is no longer misidentified as the framework type (covered by User_Defined_Task_Type_Is_Not_Treated_As_Framework_Task).

Verified IsVoid/IsAsync/IsValueTask are computed once during discovery (MemberDiscovery.cs) and consumed identically by both the delegate factory and interface-member builders, so there's no duplicated or diverging logic. Test coverage (generator snapshot + runtime DelegateMockTests) matches the described fix.

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[github-actions]

Code review: fix(mocks): emit valid lambdas for Task/ValueTask-returning delegate mocks

Verdict: Looks good — no changes requested.

I read the full diff directly (MockDelegateFactoryBuilder.cs, MockImplBuilder.cs, TypeSymbolExtensions.cs, plus the new tests/snapshot) rather than relying on prior bot summaries, and traced the generated code paths by hand.

The core fix is correct

Build() previously branched only on invokeMethod.IsVoid, which is also true for a non-generic Task/ValueTask return — routing async delegates into the Action-style branch and emitting a lambda with no return (CS1643, issue #6887). The reordered check (IsVoid && !IsAsync for true void, IsAsync for Task/Task<T>/ValueTask/ValueTask<T>) fixes this, and the new EmitAsyncBody mirrors the existing try/catch → RawReturnContext check → wrap-or-fault pattern already used for interface members (via the newly-internal EmitRawReturnCheck) instead of duplicating it. Good reuse per this repo's shared-execution-path convention.

Two bundled fixes, both verified correct and tested

• Positional lambda parameter names (arg0, arg1, …) instead of the user's declared names. I confirmed this removes a real CS0136 risk: a delegate parameter named engine, del, mock, __result, etc. would otherwise shadow a generated local. BuildArgsArray/EmitOutRefParamAssignments consistently thread the same positional mapping through, and out parameters are correctly excluded from the args array (matching the existing interface-member convention in GetArgsArrayExpression, which already filters Direction != Out) — so matcher argument counts stay aligned. Covered by Delegate_Parameter_Names_Do_Not_Collide_With_Generated_Locals.
• Namespace-qualified async-type detection (IsFrameworkGenericTask): previously named.ConstructedFrom.Name == "Task"/"ValueTask" matched any type with that simple name. Now it also requires ContainingNamespace == "System.Threading.Tasks", so a user-defined Domain.Task<T> is correctly treated as an ordinary return type. Covered by User_Defined_Task_Type_Is_Not_Treated_As_Framework_Task, and I confirmed no other generator snapshot changed as a side effect (the stricter check doesn't alter output for any existing scenario).

Previous automated review comments — addressed

Earlier bot passes on this PR (greptile) flagged the two items above as P1s; a later automated pass argued they were "pre-existing, out of scope." Having read the current diff directly, both are in fact fixed by this PR at the commit under review — the stale "pre-existing" framing in that pass appears to have mismatched the code it was citing against the diff. No further action needed since the fixes are present and tested; flagging only so it's not mistakenly reopened as an unaddressed finding.

Minor, non-blocking

• The Async_Custom_Delegate_Sets_Out_Parameter / Async_Void_Custom_Delegate_Sets_Out_Parameter tests cover the out-param + async-success path well, but there's no test combining an out/ref param with a Throws setup on an async delegate (i.e., confirming the out param is not written back when the call faults). Given the code path is a plain try/catch around the same HandleCall, this is very likely already correct — not required for this fix, just a nice-to-have follow-up.

Nothing else stood out; the change is minimal, mirrors established patterns, and the added coverage (generator snapshot + runtime DelegateMockTests) matches the described fix.