Improve error messages for keywords used as identifiers in LINQ let clauses

.github/copilot-instructions.md

@@ -26,6 +26,7 @@
 - Whitespace formatting preferences are stored in the `.editorconfig` file
 - When running `dotnet format whitespace` use the `--folder .` option followed by `--include <relative path to file>` to avoid a design-time build.
 - `dotnet format whitespace --folder . --include <relative path to file>` - Applies formatting preferences to a particular .cs or .vb file
+- **Important**: Blank lines must not contain any whitespace characters (spaces or tabs). This will cause linting errors that must be fixed.
 
 ## Code Patterns
 

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

@@ -13906,7 +13906,10 @@ private LetClauseSyntax ParseLetClause()
             Debug.Assert(this.CurrentToken.ContextualKind == SyntaxKind.LetKeyword);
             return _syntaxFactory.LetClause(
                 this.EatContextualToken(SyntaxKind.LetKeyword),
-                this.ParseIdentifierToken(),
+                // If we see a keyword followed by '=', use EatTokenAsKind to produce a better error message and recover well.
+                SyntaxFacts.IsReservedKeyword(this.CurrentToken.Kind) && this.PeekToken(1).Kind == SyntaxKind.EqualsToken
+                    ? this.EatTokenAsKind(SyntaxKind.IdentifierToken)
+                    : this.ParseIdentifierToken(),
                 this.EatToken(SyntaxKind.EqualsToken),
                 this.ParseExpressionCore());
         }

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

@@ -7330,5 +7330,50 @@ public void ObjectInitializerWithColonAndDashInsteadOfEqualsSigns()
             }
             EOF();
         }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/10446")]
+        public void LetClauseWithKeywordAsIdentifier()
+        {
+            UsingExpression("from m in methods let params = 1 select m",
+                // (1,23): error CS1041: Identifier expected; 'params' is a keyword
+                // from m in methods let params = m.GetParameters() select m
+                Diagnostic(ErrorCode.ERR_IdentifierExpectedKW, "params").WithArguments("", "params").WithLocation(1, 23));
+
+            N(SyntaxKind.QueryExpression);
+            {
+                N(SyntaxKind.FromClause);
+                {
+                    N(SyntaxKind.FromKeyword);
+                    N(SyntaxKind.IdentifierToken, "m");
+                    N(SyntaxKind.InKeyword);
+                    N(SyntaxKind.IdentifierName);
+                    {
+                        N(SyntaxKind.IdentifierToken, "methods");
+                    }
+                }
+                N(SyntaxKind.QueryBody);
+                {
+                    N(SyntaxKind.LetClause);
+                    {
+                        N(SyntaxKind.LetKeyword);
+                        M(SyntaxKind.IdentifierToken);
+                        N(SyntaxKind.EqualsToken);
+                        N(SyntaxKind.NumericLiteralExpression);
+                        {
+                            N(SyntaxKind.NumericLiteralToken, "1");
+                        }
+                    }
+                    N(SyntaxKind.SelectClause);
+                    {
+                        N(SyntaxKind.SelectKeyword);
+                        N(SyntaxKind.IdentifierName);
+                        {
+                            N(SyntaxKind.IdentifierToken, "m");
+                        }
+                    }
+                }
+            }
+            EOF();
+        }
     }
 }