Allow quoted names in ambient contexts

[RyanCavanaugh]

The node module yargs is callable (typeof require('yargs') === 'function'). It also has a property named default that is a function.

This makes writing a definition file very difficult. You'd be inclined to write

declare function yargs(): void;
declare namespace yargs {
  /* dozens of other things */
  function default(): void; // syntax error
}
export = yargs;

There's not an obvious workaround that doesn't have bad side effects (e.g. rewriting the whole mess as a var doesn't really work in the case where there's a nested class), possibly requiring a massive restructuring of the entire file.

Wherever an identifier appears in an ambient context where it could have been initialized as a property, it should be allowed to be quoted. For example

export const "default": string;
declare namespace foo {
  function "with"(): string;
}

[RyanCavanaugh]

This actually prevents some objects from being correctly represented. Consider if you had some object foo that was legal to use as

class d extends foo.var { }
let d = new foo.var();
let e = foo.default;

[RyanCavanaugh]

https://github.com/DefinitelyTyped/DefinitelyTyped/pull/17985/files#diff-ca41bc5f580b39365289711693a2ac6cR19

[BendingBender]

https://github.com/DefinitelyTyped/DefinitelyTyped/pull/18309/files#diff-c892b71507d5d1f0e96637ae3684df9eR60

[RyanCavanaugh]

DefinitelyTyped/DefinitelyTyped#21749

[mhegazy]

Note that export { x as y} is legal in a namespace in an ambient context. so the issue in the OP can be written as:

declare function yargs(): void;
declare namespace yargs {
    function _default(): void; 
    export { _default as _default };
}
export = yargs;