Refine syntax error recovery for partial code to not interfere with non-error code. (#17692)

Related to #15885
Fixes #17683

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

@@ -9413,7 +9413,7 @@ private ExpressionSyntax ParsePostFixExpression(ExpressionSyntax expr)
                         // there is a new declaration on the next line.
                         if (this.CurrentToken.TrailingTrivia.Any((int)SyntaxKind.EndOfLineTrivia) &&
                             this.PeekToken(1).Kind == SyntaxKind.IdentifierToken &&
-                            this.PeekToken(2).Kind == SyntaxKind.IdentifierToken)
+                            this.PeekToken(2).ContextualKind == SyntaxKind.IdentifierToken)
                         {
                             expr = _syntaxFactory.MemberAccessExpression(
                                 SyntaxKind.SimpleMemberAccessExpression, expr, this.EatToken(),

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

@@ -3533,5 +3533,127 @@ void M()
             EOF();
         }
 
+        [Fact, WorkItem(17683, "https://github.com/dotnet/roslyn/issues/17683")]
+        public void Bug17683a()
+        {
+            var source =
+@"from t in e
+where
+t == Int32.
+MinValue
+select t";
+            UsingExpression(source);
+            N(SyntaxKind.QueryExpression);
+            {
+                N(SyntaxKind.FromClause);
+                {
+                    N(SyntaxKind.FromKeyword);
+                    N(SyntaxKind.IdentifierToken, "t");
+                    N(SyntaxKind.InKeyword);
+                    N(SyntaxKind.IdentifierName);
+                    {
+                        N(SyntaxKind.IdentifierToken, "e");
+                    }
+                }
+                N(SyntaxKind.QueryBody);
+                {
+                    N(SyntaxKind.WhereClause);
+                    {
+                        N(SyntaxKind.WhereKeyword);
+                        N(SyntaxKind.EqualsExpression);
+                        {
+                            N(SyntaxKind.IdentifierName);
+                            {
+                                N(SyntaxKind.IdentifierToken, "t");
+                            }
+                            N(SyntaxKind.EqualsEqualsToken);
+                            N(SyntaxKind.SimpleMemberAccessExpression);
+                            {
+                                N(SyntaxKind.IdentifierName);
+                                {
+                                    N(SyntaxKind.IdentifierToken, "Int32");
+                                }
+                                N(SyntaxKind.DotToken);
+                                N(SyntaxKind.IdentifierName);
+                                {
+                                    N(SyntaxKind.IdentifierToken, "MinValue");
+                                }
+                            }
+                        }
+                    }
+                    N(SyntaxKind.SelectClause);
+                    {
+                        N(SyntaxKind.SelectKeyword);
+                        N(SyntaxKind.IdentifierName);
+                        {
+                            N(SyntaxKind.IdentifierToken, "t");
+                        }
+                    }
+                }
+            }
+            EOF();
+        }
+
+        [Fact]
+        public void Bug17683b()
+        {
+            var source =
+@"switch (e)
+{
+    case Int32.
+               MaxValue when true:
+            break;
+}";
+            UsingStatement(source);
+            N(SyntaxKind.SwitchStatement);
+            {
+                N(SyntaxKind.SwitchKeyword);
+                N(SyntaxKind.OpenParenToken);
+                N(SyntaxKind.IdentifierName);
+                {
+                    N(SyntaxKind.IdentifierToken, "e");
+                }
+                N(SyntaxKind.CloseParenToken);
+                N(SyntaxKind.OpenBraceToken);
+                N(SyntaxKind.SwitchSection);
+                {
+                    N(SyntaxKind.CasePatternSwitchLabel);
+                    {
+                        N(SyntaxKind.CaseKeyword);
+                        N(SyntaxKind.ConstantPattern);
+                        {
+                            N(SyntaxKind.SimpleMemberAccessExpression);
+                            {
+                                N(SyntaxKind.IdentifierName);
+                                {
+                                    N(SyntaxKind.IdentifierToken, "Int32");
+                                }
+                                N(SyntaxKind.DotToken);
+                                N(SyntaxKind.IdentifierName);
+                                {
+                                    N(SyntaxKind.IdentifierToken, "MaxValue");
+                                }
+                            }
+                        }
+                        N(SyntaxKind.WhenClause);
+                        {
+                            N(SyntaxKind.WhenKeyword);
+                            N(SyntaxKind.TrueLiteralExpression);
+                            {
+                                N(SyntaxKind.TrueKeyword);
+                            }
+                        }
+                        N(SyntaxKind.ColonToken);
+                    }
+                    N(SyntaxKind.BreakStatement);
+                    {
+                        N(SyntaxKind.BreakKeyword);
+                        N(SyntaxKind.SemicolonToken);
+                    }
+                }
+                N(SyntaxKind.CloseBraceToken);
+            }
+            EOF();
+        }
     }
 }
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