Fix VSTHRD110 false negative in local functions of async methods

src/Microsoft.VisualStudio.Threading.Analyzers/CommonInterest.cs

@@ -279,7 +279,7 @@ public static bool IsAwaitable(this ITypeSymbol? typeSymbol, SemanticModel seman
     {
         while (operation?.Parent is not null)
         {
-            if (operation.Parent is IAnonymousFunctionOperation or IMethodBodyOperation)
+            if (operation.Parent is IAnonymousFunctionOperation or IMethodBodyOperation or ILocalFunctionOperation)
             {
                 return operation.Parent;
             }

test/Microsoft.VisualStudio.Threading.Analyzers.Tests/VSTHRD110ObserveResultOfAsyncCallsAnalyzerTests.cs

@@ -485,4 +485,34 @@ class Class1
 
         await CSVerify.VerifyAnalyzerAsync(test);
     }
+
+    [Fact]
+    public async Task LocalVoidFunctionWithinAsyncTaskMethod()
+    {
+        string test = /* lang=c#-test */ """
+            using System.Threading.Tasks;
+
+            class Test
+            {
+                async Task DoOperationAsync()
+                {
+                    DoOperationInner();
+
+                    void DoOperationInner()
+                    {
+                        [|HelperAsync()|];
+                    }
+                }
+
+                void DoOperation()
+                {
+                    [|HelperAsync()|];
+                }
+
+                Task HelperAsync() => Task.CompletedTask;
+            }
+            """;
+
+        await CSVerify.VerifyAnalyzerAsync(test);
+    }
 }