Fix parsing of for-loops

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

@@ -9181,23 +9181,27 @@ private ForStatementSyntax ParseForStatement(SyntaxList<AttributeListSyntax> att
             var (variableDeclaration, initializers) = eatVariableDeclarationOrInitializers();
 
             // Pulled out as we need to track this when parsing incrementors to place skipped tokens.
-            SyntaxToken secondSemicolonToken;
+            var firstSemicolonToken = eatCommaOrSemicolon();
+            var condition = this.CurrentToken.Kind is not SyntaxKind.SemicolonToken and not SyntaxKind.CommaToken
+                ? this.ParseExpressionCore()
+                : null;
+            var secondSemicolonToken = eatCommaOrSemicolon();
+            // Do allow semicolons (with diagnostics) in the incrementors list.  This allows us to consume
+            // accidental extra incrementors that should have been separated by commas.
+            var incrementors = this.CurrentToken.Kind != SyntaxKind.CloseParenToken
+                ? parseForStatementExpressionList(ref secondSemicolonToken, allowSemicolonAsSeparator: true)
+                : default;
+
             var forStatement = _syntaxFactory.ForStatement(
                 attributes,
                 forToken,
                 openParen,
                 variableDeclaration,
                 initializers,
-                firstSemicolonToken: eatCommaOrSemicolon(),
-                condition: this.CurrentToken.Kind is not SyntaxKind.SemicolonToken and not SyntaxKind.CommaToken
-                    ? this.ParseExpressionCore()
-                    : null,
-                secondSemicolonToken = eatCommaOrSemicolon(),
-                // Do allow semicolons (with diagnostics) in the incrementors list.  This allows us to consume
-                // accidental extra incrementors that should have been separated by commas.
-                incrementors: this.CurrentToken.Kind != SyntaxKind.CloseParenToken
-                    ? parseForStatementExpressionList(ref secondSemicolonToken, allowSemicolonAsSeparator: true)
-                    : default,
+                firstSemicolonToken,
+                condition,
+                secondSemicolonToken,
+                incrementors,
                 eatUnexpectedTokensAndCloseParenToken(),
                 ParseEmbeddedStatement());
 

src/Compilers/CSharp/Test/Syntax/IncrementalParsing/IncrementalParsingTests.cs

@@ -3666,6 +3666,33 @@ enum VirtualKey
             WalkTreeAndVerify(tree.GetCompilationUnitRoot(), fullTree.GetCompilationUnitRoot());
         }
 
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/76439")]
+        public void InKeywordInsideAForBlock()
+        {
+            var source = """
+                void Main()
+                {
+                    for (int i = 0; i < n; i++)
+                    {
+                    }
+                }
+                """;
+            var tree = SyntaxFactory.ParseSyntaxTree(source);
+            var text = tree.GetText();
+
+            var position1 = source.IndexOf("i =") + 1;
+            var position2 = source.IndexOf("i <") + 1;
+            var position3 = source.IndexOf("i++") + 1;
+
+            text = text.WithChanges(
+                new TextChange(new TextSpan(position1, 0), "n"),
+                new TextChange(new TextSpan(position2, 0), "n"),
+                new TextChange(new TextSpan(position3, 0), "n"));
+            tree = tree.WithChangedText(text);
+            var fullTree = SyntaxFactory.ParseSyntaxTree(text.ToString());
+            WalkTreeAndVerify(tree.GetCompilationUnitRoot(), fullTree.GetCompilationUnitRoot());
+        }
+
         #endregion
 
         #region Helper functions