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

src/Microsoft.VisualStudio.Threading.Analyzers.CSharp/VSTHRD103UseAsyncOptionAnalyzer.cs

@@ -132,9 +132,19 @@ internal void AnalyzeInvocation(SyntaxNodeAnalysisContext context)
                     !methodSymbol.HasAsyncCompatibleReturnType())
                 {
                     string asyncMethodName = methodSymbol.Name + VSTHRD200UseAsyncNamingConventionAnalyzer.MandatoryAsyncSuffix;
+
+                    // For reduced extension methods (invoked as instance.Method()), look up the async
+                    // alternative on the receiver type so that extension methods defined in a separate
+                    // static class (but applicable to the receiver) are found via includeReducedExtensionMethods.
+                    // LookupSymbols with the static declaring class as container does not return extension
+                    // methods applicable to the receiver type.
+                    INamespaceOrTypeSymbol lookupContainer = methodSymbol.ReducedFrom is { } reducedFrom && reducedFrom.Parameters.Length > 0
+                        ? (INamespaceOrTypeSymbol)reducedFrom.Parameters[0].Type
+                        : methodSymbol.ContainingType;
+
                     ImmutableArray<ISymbol> symbols = context.SemanticModel.LookupSymbols(
                         invocationExpressionSyntax.Expression.GetLocation().SourceSpan.Start,
-                        methodSymbol.ContainingType,
+                        lookupContainer,
                         asyncMethodName,
                         includeReducedExtensionMethods: true);
 

test/Microsoft.VisualStudio.Threading.Analyzers.Tests/VSTHRD103UseAsyncOptionAnalyzerTests.cs

@@ -1410,6 +1410,55 @@ async Task<int> BarAsync(int id) {
         await CSVerify.VerifyAnalyzerAsync(test);
     }
 
+    [Fact]
+    public async Task SyncExtensionMethodWhereAsyncAlternativeExistsInSameStaticClassGeneratesWarning()
+    {
+        var test = @"
+using System.Threading.Tasks;
+
+public interface IExecutable { }
+
+public static class ExecutableExtensions
+{
+    public static string GetOutput(this IExecutable executable) => """";
+    public static Task<string> GetOutputAsync(this IExecutable executable) => Task.FromResult("""");
+}
+
+class Test
+{
+    async Task DoWorkAsync()
+    {
+        IExecutable exec = null!;
+        string result = exec.{|#0:GetOutput|}();
+    }
+}
+";
+
+        var withFix = @"
+using System.Threading.Tasks;
+
+public interface IExecutable { }
+
+public static class ExecutableExtensions
+{
+    public static string GetOutput(this IExecutable executable) => """";
+    public static Task<string> GetOutputAsync(this IExecutable executable) => Task.FromResult("""");
+}
+
+class Test
+{
+    async Task DoWorkAsync()
+    {
+        IExecutable exec = null!;
+        string result = await exec.GetOutputAsync();
+    }
+}
+";
+
+        DiagnosticResult expected = CSVerify.Diagnostic(Descriptor).WithLocation(0).WithArguments("GetOutput", "GetOutputAsync");
+        await CSVerify.VerifyCodeFixAsync(test, expected, withFix);
+    }
+
     private DiagnosticResult CreateDiagnostic(int line, int column, int length, string methodName)
         => CSVerify.Diagnostic(DescriptorNoAlternativeMethod).WithSpan(line, column, line, column + length).WithArguments(methodName);