Fix VSTHRD103 missing diagnostic for sync extension methods with async alternatives in the same static class

[Copilot]

VSTHRD103 failed to flag synchronous extension method calls from async methods when the *Async counterpart was defined in the same static class (e.g., exec.GetOutput() with ExecutableExtensions.GetOutputAsync(this IExecutable) in scope).

Root cause

AnalyzeInvocation called LookupSymbols(container: methodSymbol.ContainingType, ...) to find async alternatives. For reduced extension methods (instance-syntax calls like exec.GetOutput()), ContainingType is the declaring static class (e.g., ExecutableExtensions). LookupSymbols with a static class as container returns nothing useful — it looks for instance-accessible members, but static classes have no instances and their extension methods are scoped to other receiver types.

Fix

• VSTHRD103UseAsyncOptionAnalyzer.cs — When methodSymbol.ReducedFrom is non-null (reduced extension method), use reducedFrom.Parameters[0].Type (the receiver type, e.g., IExecutable) as the LookupSymbols container instead of ContainingType. With includeReducedExtensionMethods: true, this correctly finds extension methods applicable to the receiver type across all in-scope static classes.

• VSTHRD103UseAsyncOptionAnalyzerTests.cs — Added SyncExtensionMethodWhereAsyncAlternativeExistsInSameStaticClassGeneratesWarning covering the repro scenario from the issue, including code fix verification.

// Before fix: no diagnostic on exec.GetOutput()
// After fix: VSTHRD103 flags GetOutput, code fix produces:
string result = await exec.GetOutputAsync();

Warning

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Pull request overview

Fixes a gap in VSTHRD103 where synchronous reduced extension method invocations (instance-syntax) were not producing diagnostics when an *Async alternative existed as an extension method in the same (or any in-scope) static class.

Changes:

• Updates VSTHRD103’s async-alternative lookup to use the reduced extension method’s receiver type as the LookupSymbols container (instead of the declaring static class).
• Adds a regression test covering the scenario where both sync/async extension methods live in the same static class and the sync one is invoked via instance syntax.
• Verifies the code fix produces await receiver.MethodAsync() as expected.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Microsoft.VisualStudio.Threading.Analyzers.CSharp/VSTHRD103UseAsyncOptionAnalyzer.cs	Adjusts symbol lookup to correctly find async alternatives for reduced extension method invocations.
test/Microsoft.VisualStudio.Threading.Analyzers.Tests/VSTHRD103UseAsyncOptionAnalyzerTests.cs	Adds a regression test + code-fix verification for the same-static-class extension method repro.