Fix parsing of scoped before this in parameter list

[Copilot]

Fix parsing of scoped before this in parameter list ✅

Summary

Fixed a parser bug where scoped in this int failed to parse even though the ordering of this keyword was supposed to be relaxed according to LDM notes.

Changes Made

Parser Fix (LanguageParser.cs:4884):

-// Look if ref/out/in/readonly are next
-while (this.CurrentToken.Kind is SyntaxKind.RefKeyword or SyntaxKind.OutKeyword or SyntaxKind.InKeyword or SyntaxKind.ReadOnlyKeyword)
+// Look if ref/out/in/readonly/this are next
+while (this.CurrentToken.Kind is SyntaxKind.RefKeyword or SyntaxKind.OutKeyword or SyntaxKind.InKeyword or SyntaxKind.ReadOnlyKeyword or SyntaxKind.ThisKeyword)

Parsing Tests Added (DeclarationScopeParsingTests.cs):

• ✅ ScopedWithThis_01: scoped in this int (main failing case)
• ✅ ScopedWithThis_02: scoped ref this int
• ✅ ScopedWithThis_03: scoped ref readonly this int
• ✅ ScopedWithThis_04: this scoped in int (regression test)

Attribute Tests Added (AttributeTests_LifetimeAnnotation.cs):

• ✅ EmitAttribute_ExtensionMethodParameters_ScopedInThis
• ✅ EmitAttribute_ExtensionMethodParameters_ScopedRefThis
• ✅ EmitAttribute_ExtensionMethodParameters_ScopedRefReadonlyThis

Test Results

✅ All new parsing tests pass (4/4)
✅ All new attribute tests pass (3/3)
✅ All existing tests pass

• AttributeTests_ScopedRef: 19 passed (16 existing + 3 new)
• DeclarationScopeParsingTests: 476 passed
• Overall Syntax.UnitTests: 9,733 passed

Manual Verification

All issue scenarios now work correctly:

void M(in this int p);           // ✅ Works (before and after)
void M(this in int p);           // ✅ Works (before and after)
void M(scoped in this int p);    // ✅ NOW WORKS (failed before)
void M(this scoped in int p);    // ✅ Works (before and after)

Impact

Minimal surgical change - one line modified in parser logic to support the relaxed ordering of parameter modifiers as intended by the language design.

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[jcouv]

@copilot Remove extra empty lines below. And for the scenarios you've added above, add corresponding tests to AttributeTests_ScopedRef for the scenarios that aren't already covered.

[Copilot]

Done. Removed the extra empty lines and added three attribute tests for the extension method scenarios with this keyword combinations in commit af5f4ec.