[ILLink] Add regression test for KeyNotFoundException crash in async partial methods with capturing lambdas

[Copilot]

Roslyn does not emit [AsyncStateMachineAttribute] on async partial methods. ILLink's TryGetStateMachineType relies exclusively on this attribute, so the state machine type is never added to generatedTypeToTypeArgs. When a capturing lambda inside MoveNext creates a display class whose generic parameter's Owner is the missing state machine type, MapGeneratedTypeTypeParameters crashes with KeyNotFoundException.

Changes

• CompilerGeneratedTypes.cs — Added PartialAsyncMethodWithLambda regression test: a nested partial class with a generic async partial method that captures a lambda referencing the generic type parameter. This test reproduces the crash described in the linked issue.

partial class PartialAsyncMethodWithLambda
{
    public partial Task<T> GetData<T>() where T : new();
}
partial class PartialAsyncMethodWithLambda
{
    public async partial Task<T> GetData<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicMethods)] T>() where T : new()
    {
        var tcs = new TaskCompletionSource<T>();
        Action<T> callback = (result) =>
        {
            _ = typeof(T).GetMethods();
            tcs.TrySetResult(result);
        };
        callback(new T());
        return await tcs.Task;
    }
}

Note

This PR was authored with the assistance of GitHub Copilot.

Original prompt

[Chronological Review] 1. Initial request: User wanted to reproduce the ILLink trimmer crash from #122800 - a KeyNotFoundException in MapGeneratedTypeTypeParameters when processing compiler-generated async state machine types for generic methods.

2. Exploration phase: Read the issue, related issues ([area-Tools-ILLink]: Key Not Found: 'QuickGraph.Collections.FibonacciHeap`2/<GetEnumerator>d__8' #109821), Kebechet/Maui.Health fix commits, and deeply analyzed CompilerGeneratedState.cs (the buggy code), CompilerGeneratedNames.cs (name matching), and existing test cases.

3. First repro attempts: Created test project at C:\Users\angocke\tmp\link-crash with ReproLib (class library), ReproApp (console with trimming), and Inspector (reflection tool). Multiple attempts with generic async methods + lambdas did NOT crash the trimmer.

4. Key breakthrough: Downloaded the actual Maui.Health NuGet package (v2.0.0-preview9, the pre-fix version) and inspected it with Cecil. Discovered that GetHealthData and WriteHealthData are async methods with state machines but are MISSING [AsyncStateMachineAttribute]. Only GetCumulativeHealthDataAsync (a non-partial async method) had the attribute.

5. Root cause identified: Roslyn does NOT emit [AsyncStateMachineAttribute] on async partial methods. ILLink's TryGetStateMachineType relies exclusively on this attribute to discover state machines. Without it, the state machine type is never added to generatedTypeToTypeArgs, but the display class (created by a lambda in MoveNext) IS added — and its generic parameter's Owner references the missing state machine, causing KeyNotFoundException.

6. Minimal repro created: A partial method declaration + async partial implementation with a generic-capturing lambda. Confirmed crash: KeyNotFoundException: The given key 'ReproLib.Service/<GetData>d__01' was not present in the dictionary.`

7. Test case added: Added the repro as a test in CompilerGeneratedTypes.cs in the ILLink test suite.

8. Test validation: Tried running tests. Analyzer tests (ILLink.RoslynAnalyzer.Tests) built and passed (2/2). The full linker tests (Mono.Linker.Tests) couldn't build due to missing VS C++ tools on the machine, and the prebuilt test runner had a pre-existing version mismatch error (ProducedBy not found) unrelated to the changes.

9. Most recent: User said "cd ~/code/runtime", agent changed directory. Then user requested conversation summary.

[Intent Mapping]

• Primary: Reproduce the user error from [area-Tools-ILLink]: The given key 'Maui.Health.Services.HealthService/<WriteHealthData>d__17`1' was not present in the dictionary #122800
• Secondary: Create minimal repro of the exact Roslyn pattern
• Tertiary: Add as a test case in ~/code/runtime/src/illink

[Technical Inventory]

• ILLink trimmer (IL Linker) - the .NET assembly trimming tool
• CompilerGeneratedState.cs - core file with the bug
• Mono.Cecil - used for IL inspection
• Roslyn compiler - generates the problematic codegen for async partial methods
• .NET 11.0.100-preview.2 SDK (on this machine), bug reported on .NET 10.0.101

[Code Archaeology]

• CompilerGeneratedState.cs: MapGeneratedTypeTypeParameters (lines 323-375), ProcessMethod (lines 131-221), TryGetStateMachineType (lines 82-102), ScanForInit (lines 377-418)
• CompilerGeneratedNames.cs: IsStateMachineType, IsLambdaDisplayClass, IsStateMachineOrDisplayClass
• CompilerGeneratedTypes.cs test file: Added PartialAsyncMethodWithLambda test class
• Repro files at C:\Users\angocke\tmp\link-crash\

[Progress Assessment]

• DONE: Root cause identified
• DONE: Minimal repro created and validated
• DONE: Test case added to CompilerGeneratedTypes.cs
• DONE: Analyzer tests pass with the new test
• NOT POSSIBLE on this machine: Full linker test run (needs VS C++ tools)

[Context Validation]
All critical information captured including exact file paths, line numbers, crash mechanism, and test code.

[Recent Commands Analysis]
The most recent commands were:

1. cd C:\Users\angocke\code\runtime; Get-Location - Changed to the runtime directory
2. Before that: git --no-pager diff showing the final diff of CompilerGeneratedTypes.cs
3. Before that: Running analyzer tests (passed 2/2) and linker tests (failed due to pre-existing env issue)
4. The user then asked for a conversation summary

1. Conversation Overview: - Primary Objectives: "I'd like to repro the user error in https://github.com//issues/122800" — reproduce the ILLink trimmer crash (KeyNotFoundException in MapGeneratedTypeTypeParameters) and "add this as a test case in ~/code/runtime/src/illink" - Session Context: Started with issue analysis → explored ILLink source code → created repro project → multiple failed attempts → downloaded actual Maui.Health NuGet and inspected IL with Cecil → discovered root cause (missing AsyncStateMachineAttribute on async partial methods) → created minimal repro → confirmed crash → added test case to CompilerGeneratedTypes.cs → validated with analyzer tests - User Intent Evolution: Started as "reproduce the crash" → evolved to "add as test case" after repro was achieved

2. Technical Foundat...

Created from Copilot CLI via the copilot delegate command.

[agocke]

@copilot Please revert the fix for now, just add the test

[Copilot]

@copilot Please revert the fix for now, just add the test

Done in a8f7794. CompilerGeneratedState.cs is reverted to its original state; only the regression test in CompilerGeneratedTypes.cs remains.