Fix VSTHRD104 false positives

src/Microsoft.VisualStudio.Threading.Analyzers.CSharp/VSTHRD104OfferAsyncOptionAnalyzer.cs

@@ -98,12 +98,101 @@ private void InspectMemberAccess(SyntaxNodeAnalysisContext context, MemberAccess
                 {
                     if (item.Method.IsMatch(invokedMember))
                     {
+                        if (invokedMember is IPropertySymbol { Name: nameof(Task<int>.Result), ContainingType: { } containingType }
+                            && Utils.IsTask(containingType)
+                            && this.IsGuardedBySuccessfulCompletion(context, memberAccessSyntax))
+                        {
+                            return;
+                        }
+
                         Location? location = memberAccessSyntax.Name.GetLocation();
                         context.ReportDiagnostic(Diagnostic.Create(Descriptor, location));
                         this.diagnosticReported = true;
                     }
                 }
             }
         }
+
+        private bool IsGuardedBySuccessfulCompletion(SyntaxNodeAnalysisContext context, MemberAccessExpressionSyntax resultAccess)
+        {
+            ISymbol? taskSymbol = context.SemanticModel.GetSymbolInfo(resultAccess.Expression, context.CancellationToken).Symbol;
+            if (taskSymbol is not ILocalSymbol and not IParameterSymbol)
+            {
+                return false;
+            }
+
+            foreach (IfStatementSyntax ifStatement in resultAccess.Ancestors()
+                .TakeWhile(ancestor => ancestor is not LocalFunctionStatementSyntax)
+                .OfType<IfStatementSyntax>())
+            {
+                if (ifStatement.Statement.Span.Contains(resultAccess.Span)
+                    && ConditionProvesSuccessfulCompletion(ifStatement.Condition, taskSymbol)
+                    && !MayWriteSymbolBefore(ifStatement.Statement, resultAccess, taskSymbol))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+
+            bool ConditionProvesSuccessfulCompletion(ExpressionSyntax condition, ISymbol expectedTaskSymbol)
+            {
+                while (condition is ParenthesizedExpressionSyntax parenthesized)
+                {
+                    condition = parenthesized.Expression;
+                }
+
+                if (condition is BinaryExpressionSyntax { RawKind: (int)SyntaxKind.LogicalAndExpression } logicalAnd)
+                {
+                    return ConditionProvesSuccessfulCompletion(logicalAnd.Left, expectedTaskSymbol)
+                        || ConditionProvesSuccessfulCompletion(logicalAnd.Right, expectedTaskSymbol);
+                }
+
+                if (condition is not MemberAccessExpressionSyntax { Name.Identifier.ValueText: "IsCompletedSuccessfully" } completionAccess)
+                {
+                    return false;
+                }
+
+                ISymbol? completionProperty = context.SemanticModel.GetSymbolInfo(completionAccess, context.CancellationToken).Symbol;
+                ISymbol? completedTaskSymbol = context.SemanticModel.GetSymbolInfo(completionAccess.Expression, context.CancellationToken).Symbol;
+                return completionProperty is IPropertySymbol { ContainingType: { } containingType }
+                    && Utils.IsTask(containingType)
+                    && SymbolEqualityComparer.Default.Equals(expectedTaskSymbol, completedTaskSymbol);
+            }
+
+            bool MayWriteSymbolBefore(StatementSyntax statement, ExpressionSyntax expression, ISymbol expectedTaskSymbol)
+            {
+                foreach (SyntaxNode node in statement.DescendantNodes().Where(node => node.SpanStart < expression.SpanStart))
+                {
+                    if (node is RefExpressionSyntax refExpression
+                        && SymbolEqualityComparer.Default.Equals(
+                            context.SemanticModel.GetSymbolInfo(refExpression.Expression, context.CancellationToken).Symbol,
+                            expectedTaskSymbol))
+                    {
+                        return true;
+                    }
+
+                    ExpressionSyntax? writtenExpression = node switch
+                    {
+                        AssignmentExpressionSyntax assignment => assignment.Left,
+                        PrefixUnaryExpressionSyntax prefix when prefix.IsKind(SyntaxKind.PreIncrementExpression) || prefix.IsKind(SyntaxKind.PreDecrementExpression) => prefix.Operand,
+                        PostfixUnaryExpressionSyntax postfix when postfix.IsKind(SyntaxKind.PostIncrementExpression) || postfix.IsKind(SyntaxKind.PostDecrementExpression) => postfix.Operand,
+                        ArgumentSyntax argument when argument.RefOrOutKeyword.IsKind(SyntaxKind.RefKeyword) || argument.RefOrOutKeyword.IsKind(SyntaxKind.OutKeyword) => argument.Expression,
+                        _ => null,
+                    };
+
+                    if (writtenExpression is not null && writtenExpression.DescendantNodesAndSelf()
+                        .OfType<ExpressionSyntax>()
+                        .Any(candidate => SymbolEqualityComparer.Default.Equals(
+                            context.SemanticModel.GetSymbolInfo(candidate, context.CancellationToken).Symbol,
+                            expectedTaskSymbol)))
+                    {
+                        return true;
+                    }
+                }
+
+                return false;
+            }
+        }
     }
 }

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

@@ -324,8 +324,27 @@ public static bool IsLazyOfT([NotNullWhen(true)] INamedTypeSymbol? constructedTy
     public static bool HasAsyncAlternative(this IMethodSymbol methodSymbol, CancellationToken cancellationToken)
     {
         cancellationToken.ThrowIfCancellationRequested();
-        return methodSymbol.ContainingType.GetMembers(methodSymbol.Name + VSTHRD200UseAsyncNamingConventionAnalyzer.MandatoryAsyncSuffix)
-            .Any(alt => IsXAtLeastAsPublicAsY(alt, methodSymbol));
+        string asyncMethodName = methodSymbol.Name + VSTHRD200UseAsyncNamingConventionAnalyzer.MandatoryAsyncSuffix;
+        if (methodSymbol.ContainingType.GetMembers(asyncMethodName).Any(alt => IsXAtLeastAsPublicAsY(alt, methodSymbol)))
+        {
+            return true;
+        }
+
+        if (methodSymbol.IsExtensionMethod && !methodSymbol.Parameters.IsDefaultOrEmpty && methodSymbol.Parameters[0].Type is INamedTypeSymbol receiverType)
+        {
+            IEnumerable<ISymbol> receiverMembers = receiverType.TypeKind == TypeKind.Interface
+                ? receiverType.AllInterfaces.SelectMany(iface => iface.GetMembers(asyncMethodName))
+                : Enumerable.Empty<ISymbol>();
+            for (INamedTypeSymbol? currentType = receiverType; currentType is not null; currentType = currentType.BaseType)
+            {
+                receiverMembers = receiverMembers.Concat(currentType.GetMembers(asyncMethodName));
+            }
+
+            return receiverMembers
+                .Any(alt => IsXAtLeastAsPublicAsY(alt, methodSymbol));
+        }
+
+        return false;
     }
 
     public static bool IsXAtLeastAsPublicAsY(ISymbol x, ISymbol y)