Fix parsing an expression-bodied property with async as the type (#25704)

src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

@@ -2316,7 +2316,7 @@ private MemberDeclarationSyntax ParseMemberDeclarationOrStatement(SyntaxKind par
                 if (!sawRef &&
                     identifierOrThisOpt != null &&
                     (typeParameterListOpt != null && typeParameterListOpt.ContainsDiagnostics
-                      || this.CurrentToken.Kind != SyntaxKind.OpenParenToken && this.CurrentToken.Kind != SyntaxKind.OpenBraceToken) &&
+                      || this.CurrentToken.Kind != SyntaxKind.OpenParenToken && this.CurrentToken.Kind != SyntaxKind.OpenBraceToken && this.CurrentToken.Kind != SyntaxKind.EqualsGreaterThanToken) &&
                     ReconsiderTypeAsAsyncModifier(ref modifiers, ref type, ref explicitInterfaceOpt, identifierOrThisOpt, typeParameterListOpt))
                 {
                     goto parse_member_name;

src/Compilers/CSharp/Test/Syntax/Parsing/AsyncParsingTests.cs

@@ -1698,5 +1698,246 @@ public void AsyncGenericType()
             }
             EOF();
         }
+
+        [Fact]
+        [WorkItem(16044, "https://github.com/dotnet/roslyn/issues/16044")]
+        public void AsyncAsType_Property_ExpressionBody()
+        {
+            UsingTree("class async { async async => null; }").GetDiagnostics().Verify(
+                // (1,27): error CS8026: Feature 'expression-bodied property' is not available in C# 5. Please use language version 6 or greater.
+                // class async { async async => null; }
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion5, "=> null").WithArguments("expression-bodied property", "6").WithLocation(1, 27)
+                );
+
+            N(SyntaxKind.CompilationUnit);
+            {
+                N(SyntaxKind.ClassDeclaration);
+                {
+                    N(SyntaxKind.ClassKeyword);
+                    N(SyntaxKind.IdentifierToken, "async");
+                    N(SyntaxKind.OpenBraceToken);
+                    N(SyntaxKind.PropertyDeclaration);
+                    {
+                        N(SyntaxKind.IdentifierName);
+                        {
+                            N(SyntaxKind.IdentifierToken, "async");
+                        }
+                        N(SyntaxKind.IdentifierToken, "async");
+                        N(SyntaxKind.ArrowExpressionClause);
+                        {
+                            N(SyntaxKind.EqualsGreaterThanToken);
+                            N(SyntaxKind.NullLiteralExpression);
+                            {
+                                N(SyntaxKind.NullKeyword);
+                            }
+                        }
+                        N(SyntaxKind.SemicolonToken);
+                    }
+                    N(SyntaxKind.CloseBraceToken);
+                }
+                N(SyntaxKind.EndOfFileToken);
+            }
+            EOF();
+        }
+
+        [Fact]
+        [WorkItem(16044, "https://github.com/dotnet/roslyn/issues/16044")]
+        public void AsyncAsType_Property()
+        {
+            UsingTree("class async { async async { get; } }").GetDiagnostics().Verify();
+            N(SyntaxKind.CompilationUnit);
+            {
+                N(SyntaxKind.ClassDeclaration);
+                {
+                    N(SyntaxKind.ClassKeyword);
+                    N(SyntaxKind.IdentifierToken, "async");
+                    N(SyntaxKind.OpenBraceToken);
+                    N(SyntaxKind.PropertyDeclaration);
+                    {
+                        N(SyntaxKind.IdentifierName);
+                        {
+                            N(SyntaxKind.IdentifierToken, "async");
+                        }
+                        N(SyntaxKind.IdentifierToken, "async");
+                        N(SyntaxKind.AccessorList);
+                        {
+                            N(SyntaxKind.OpenBraceToken);
+                            N(SyntaxKind.GetAccessorDeclaration);
+                            {
+                                N(SyntaxKind.GetKeyword);
+                                N(SyntaxKind.SemicolonToken);
+                            }
+                            N(SyntaxKind.CloseBraceToken);
+                        }
+                    }
+                    N(SyntaxKind.CloseBraceToken);
+                }
+                N(SyntaxKind.EndOfFileToken);
+            }
+            EOF();
+        }
+
+        [Fact]
+        [WorkItem(16044, "https://github.com/dotnet/roslyn/issues/16044")]
+        public void AsyncAsType_Indexer_ExpressionBody_ErrorCase()
+        {
+            UsingTree("interface async { async this[async i] => null; }").GetDiagnostics().Verify(
+                // (1,39): error CS8026: Feature 'expression-bodied indexer' is not available in C# 5. Please use language version 6 or greater.
+                // interface async { async this[async i] => null; }
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion5, "=> null").WithArguments("expression-bodied indexer", "6").WithLocation(1, 39)
+                );
+
+            N(SyntaxKind.CompilationUnit);
+            {
+                N(SyntaxKind.InterfaceDeclaration);
+                {
+                    N(SyntaxKind.InterfaceKeyword);
+                    N(SyntaxKind.IdentifierToken, "async");
+                    N(SyntaxKind.OpenBraceToken);
+                    N(SyntaxKind.IndexerDeclaration);
+                    {
+                        N(SyntaxKind.IdentifierName);
+                        {
+                            N(SyntaxKind.IdentifierToken, "async");
+                        }
+                        N(SyntaxKind.ThisKeyword);
+                        N(SyntaxKind.BracketedParameterList);
+                        {
+                            N(SyntaxKind.OpenBracketToken);
+                            N(SyntaxKind.Parameter);
+                            {
+                                N(SyntaxKind.IdentifierName);
+                                {
+                                    N(SyntaxKind.IdentifierToken, "async");
+                                }
+                                N(SyntaxKind.IdentifierToken, "i");
+                            }
+                            N(SyntaxKind.CloseBracketToken);
+                        }
+                        N(SyntaxKind.ArrowExpressionClause);
+                        {
+                            N(SyntaxKind.EqualsGreaterThanToken);
+                            N(SyntaxKind.NullLiteralExpression);
+                            {
+                                N(SyntaxKind.NullKeyword);
+                            }
+                        }
+                        N(SyntaxKind.SemicolonToken);
+                    }
+                    N(SyntaxKind.CloseBraceToken);
+                }
+                N(SyntaxKind.EndOfFileToken);
+            }
+            EOF();
+        }
+
+        [Fact]
+        [WorkItem(16044, "https://github.com/dotnet/roslyn/issues/16044")]
+        public void AsyncAsType_Indexer()
+        {
+            UsingTree("interface async { async this[async i] { get; } }").GetDiagnostics().Verify();
+            N(SyntaxKind.CompilationUnit);
+            {
+                N(SyntaxKind.InterfaceDeclaration);
+                {
+                    N(SyntaxKind.InterfaceKeyword);
+                    N(SyntaxKind.IdentifierToken, "async");
+                    N(SyntaxKind.OpenBraceToken);
+                    N(SyntaxKind.IndexerDeclaration);
+                    {
+                        N(SyntaxKind.IdentifierName);
+                        {
+                            N(SyntaxKind.IdentifierToken, "async");
+                        }
+                        N(SyntaxKind.ThisKeyword);
+                        N(SyntaxKind.BracketedParameterList);
+                        {
+                            N(SyntaxKind.OpenBracketToken);
+                            N(SyntaxKind.Parameter);
+                            {
+                                N(SyntaxKind.IdentifierName);
+                                {
+                                    N(SyntaxKind.IdentifierToken, "async");
+                                }
+                                N(SyntaxKind.IdentifierToken, "i");
+                            }
+                            N(SyntaxKind.CloseBracketToken);
+                        }
+                        N(SyntaxKind.AccessorList);
+                        {
+                            N(SyntaxKind.OpenBraceToken);
+                            N(SyntaxKind.GetAccessorDeclaration);
+                            {
+                                N(SyntaxKind.GetKeyword);
+                                N(SyntaxKind.SemicolonToken);
+                            }
+                            N(SyntaxKind.CloseBraceToken);
+                        }
+                    }
+                    N(SyntaxKind.CloseBraceToken);
+                }
+                N(SyntaxKind.EndOfFileToken);
+            }
+            EOF();
+        }
+
+        [Fact]
+        [WorkItem(16044, "https://github.com/dotnet/roslyn/issues/16044")]
+        public void AsyncAsType_Property_ExplicitInterface()
+        {
+            UsingTree("class async : async { async async.async => null; }").GetDiagnostics().Verify(
+                // (1,41): error CS8026: Feature 'expression-bodied property' is not available in C# 5. Please use language version 6 or greater.
+                // class async : async { async async.async => null; }
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion5, "=> null").WithArguments("expression-bodied property", "6").WithLocation(1, 41)
+                );
+            N(SyntaxKind.CompilationUnit);
+            {
+                N(SyntaxKind.ClassDeclaration);
+                {
+                    N(SyntaxKind.ClassKeyword);
+                    N(SyntaxKind.IdentifierToken, "async");
+                    N(SyntaxKind.BaseList);
+                    {
+                        N(SyntaxKind.ColonToken);
+                        N(SyntaxKind.SimpleBaseType);
+                        {
+                            N(SyntaxKind.IdentifierName);
+                            {
+                                N(SyntaxKind.IdentifierToken, "async");
+                            }
+                        }
+                    }
+                    N(SyntaxKind.OpenBraceToken);
+                    N(SyntaxKind.PropertyDeclaration);
+                    {
+                        N(SyntaxKind.IdentifierName);
+                        {
+                            N(SyntaxKind.IdentifierToken, "async");
+                        }
+                        N(SyntaxKind.ExplicitInterfaceSpecifier);
+                        {
+                            N(SyntaxKind.IdentifierName);
+                            {
+                                N(SyntaxKind.IdentifierToken, "async");
+                            }
+                            N(SyntaxKind.DotToken);
+                        }
+                        N(SyntaxKind.IdentifierToken, "async");
+                        N(SyntaxKind.ArrowExpressionClause);
+                        {
+                            N(SyntaxKind.EqualsGreaterThanToken);
+                            N(SyntaxKind.NullLiteralExpression);
+                            {
+                                N(SyntaxKind.NullKeyword);
+                            }
+                        }
+                        N(SyntaxKind.SemicolonToken);
+                    }
+                    N(SyntaxKind.CloseBraceToken);
+                }
+                N(SyntaxKind.EndOfFileToken);
+            }
+            EOF();
+        }
     }
 }