Allow accessor properties in ambient contexts when targeting ES5

[Copilot]

This PR fixes an issue where TypeScript incorrectly disallowed accessor properties in ambient contexts (.d.ts files, declare class, declare module, etc.) when targeting ES5.

Problem

Previously, TypeScript would error on accessor properties in all contexts when targeting ES5:

// @target: es5

// This would error even though it's ambient (no runtime code generated)
declare class LibraryClass {
    accessor options: any; // ❌ Error: Properties with the 'accessor' modifier are only available when targeting ECMAScript 2015 and higher
}

// This would also error
declare module "some-library" {
    export class Parser {
        accessor config: any; // ❌ Same error
    }
}

// This should still error (runtime code needs to be generated)
class UserClass {
    accessor prop: string; // ❌ Should still error
}

This forced consumers of libraries like @babel/parser (which use accessor in their .d.ts files) to enable skipLibCheck when targeting ES5, which is not ideal.

Solution

Modified the checkGrammarProperty function in src/compiler/checker.ts to exclude ambient contexts from the ES5 accessor restriction:

// Before
if (languageVersion < ScriptTarget.ES2015 && isAutoAccessorPropertyDeclaration(node)) {
    return grammarErrorOnNode(node.name, Diagnostics.Properties_with_the_accessor_modifier_are_only_available_when_targeting_ECMAScript_2015_and_higher);
}

// After  
if (languageVersion < ScriptTarget.ES2015 && isAutoAccessorPropertyDeclaration(node) && !(node.flags & NodeFlags.Ambient)) {
    return grammarErrorOnNode(node.name, Diagnostics.Properties_with_the_accessor_modifier_are_only_available_when_targeting_ECMAScript_2015_and_higher);
}

Result

With this fix:

// @target: es5

// ✅ No longer errors - ambient contexts are allowed
declare class LibraryClass {
    accessor options: any;
    static accessor defaults: any;
}

declare module "some-library" {
    export class Parser {
        accessor config: any;
    }
}

// ❌ Still correctly errors - regular classes targeting ES5 cannot use accessor
class UserClass {
    accessor prop: string; // Error: Properties with the 'accessor' modifier are only available when targeting ECMAScript 2015 and higher
}

Testing

• Added comprehensive test case accessorInAmbientContextES5.ts covering all ambient context scenarios
• Verified all existing autoAccessor tests continue to pass (252 tests)
• Confirmed regular classes still error correctly when targeting ES5
• No regressions detected in broader test runs

This change follows the same pattern used elsewhere in TypeScript for handling ES2015+ features in ambient contexts and maintains backward compatibility.

Fixes #61181.

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[RyanCavanaugh]

Fix your line endings in checker.ts to match the rest of the file

[Copilot]

Fix your line endings in checker.ts to match the rest of the file

Fixed line endings to match CRLF format used throughout checker.ts file. c8adde3

[RyanCavanaugh]

🏆