Infer RegExp for regular expression literals under isolatedDeclarations - #65419

Fixes #58383

Problem

Under --isolatedDeclarations, a declaration initialized with a regular expression literal reports TS9010/TS9012/TS9013 even though the type can only ever be RegExp:

export const re = /ab+c/gi;
//           ~~ error TS9010: Variable must have an explicit type annotation with --isolatedDeclarations.

typeFromExpression in the pseudochecker has cases for numeric, string, bigint and boolean literals but none for RegularExpressionLiteral, so it falls through to "needs inference". The checker itself always types the literal as the global RegExp (checkRegularExpressionLiteral).

Change

• pseudochecker: a PseudoTypeKindRegExp kind (a singleton like PseudoTypeString) and a RegularExpressionLiteral case returning it. There are no regex literal types, so it is not wrapped in MaybeConstLocation: the result is RegExp in const contexts too.
• pseudotypenodebuilder: the new kind serializes as typeToTypeNode(globalRegExpType) and maps to globalRegExpType in pseudoTypeToType.

Serializing the global type instead of a hand-made RegExp identifier is what handles the shadowing concern raised in the issue: when a file declares its own RegExp, the node builder emits globalThis.RegExp, the same way declaration emit already refers to any shadowed global. That stays syntactically determinable for isolated tools, since the shadowing declaration or import is in the same file.

Notes

• With noLib, globalRegExpType is the empty object type, so the emitted type is {}, the same as non-isolated declaration emit in that configuration.
• This only changes the Go compiler; the 5.x (strada) line is unaffected.

Tests

isolatedDeclarationsRegExp.ts covers const/let/var, object literal properties, an as const object with a regex array, a function and an arrow return, class fields (including an optional one), parameter defaults followed by a required parameter, and two files that shadow RegExp (a class and a type alias). Output:

export declare const re: RegExp;
export declare const constObj: {
    readonly pattern: RegExp;
    readonly list: readonly [RegExp, RegExp];
};
export declare function withDefault(pattern: RegExp | undefined, flags: string): void;
// shadowed.d.ts
declare class RegExp { exec(): void; }
export declare const local: RegExp;
export declare const global: globalThis.RegExp;
// typeShadowed.d.ts
type RegExp = string;
export declare const globalType: globalThis.RegExp;

Without the change the same test reports 17 isolatedDeclarations errors (TS9010, TS9012, TS9013, TS9025) and no declarations for those initializers. No other baselines change. npx hereby test, lint, check:format and tidy pass with no diff.

I used Claude Code to draft this change; I reviewed and tested every line and will handle review feedback myself.

🤖 Generated with Claude Code