Reject using/await using declarations in switch case/default clauses

[PaulyBearCoding]

Summary

Implements parser validation to reject using and await using declarations when directly nested within case or default clauses of switch statements, per updated ECMAScript spec.

Fixes #62708

Background

The ECMAScript spec was updated (rbuckton/ecma262#14) to disallow using declarations in switch case/default clauses. All major JS engines have agreed to implement this restriction.

Rationale:

• Makes the number of resource declarations statically knowable
• Simplifies spec (removes special cases in HasCallInTailPosition and HasUnterminatedUsingDeclaration)
• Pattern is rarely used in practice ("obscure, at best" per spec authors)

Related implementations:

• Babel: Disallow using in bare case statement babel/babel#17323
• Acorn: fix: raises syntax errors on invalid using declarations acornjs/acorn#1374

Changes

1. Parser Validation

• Modified parseVariableDeclarationList() in src/compiler/parser.ts
• Check if parsingContext === ParsingContext.SwitchClauseStatements
• Report error TS95198 for both using and await using

2. Diagnostic Message

• Added new error message (code 95198) in src/compiler/diagnosticMessages.json
• Format: A '{0}' declaration cannot be placed within a 'case' or 'default' clause.

3. Test Baseline

• Added error baseline for tests/cases/conformance/statements/VariableStatements/usingDeclarations/usingDeclarations.1.ts
• Lines 100, 104, 111 now correctly produce TS95198 errors

Valid Workaround

Users can wrap using declarations in a block statement:

// ❌ Error
switch (x) {
  case 0:
    using resource = ...;  // Error TS95198
    break;
}

// ✅ Valid
switch (x) {
  case 0: {
    using resource = ...;  // OK - wrapped in block
    break;
  }
}

Testing

• Added error baseline for existing test case
• Verified parser correctly rejects using in case clauses
• Verified parser correctly rejects await using in case clauses
• Verified parser correctly rejects using in default clauses
• Verified valid code (block-wrapped, outside switch) still compiles

Checklist

• Parser validation implemented
• Diagnostic message added
• Test baselines updated
• Clear error message with helpful guidance
• No false positives (valid code unaffected)

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