Can't evaluate equality of symbols made by Symbol.for

This is also an obstacle of branded types and ghost types. I think resolving this problem is easy because these types are just a new literal type similar to string literal types.

TypeScript Version: 3.7.x-dev.20191228

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Code

const a = Symbol.for('');
const b = Symbol.for('');
const c: typeof b = Symbol.for(''); // error
a === b; // error

Expected behavior: no error Actual behavior: Type ‘typeof b’ is not assignable to type ‘typeof a’. This condition will always return ‘false’ since the types ‘typeof a’ and ‘typeof b’ have no overlap. Playground Link: https://www.typescriptlang.org/play/index.html?ts=3.8.0-dev.20191228&ssl=1&ssc=1&pln=4&pc=18#code/MYewdgzgLgBAhjAvDAygTwLYCMQBsB0AZiAE4AUA5BQJQDcAUKJLFkqpjgceVXY+NBjAAXDChoADgFMQhGK2TpseIqUo1aMAPRaYUkiVL0EiU-M069B0kA

Related Issues: #35200

[pjeby]

Throwing in a use case here: I’ve got a library that exports a symbol for purposes of interop, that other code can declare methods with that symbol, without needing to depend on the library that defines the symbol. But then, in a project that uses both of those libraries, Typescript can’t tell that module 2 implements the interface specified by module 1, even though it uses the same Symbol.for().

As it stands, the only way to get this kind of interop is with strings, not symbols, as there’s no “global symbol” or “symbol for” syntax in Typescript. Using typeof can’t work here because that would force the common dependency, and a shared type definitions file, since merely equivalent types are not sufficient for “unique symbol” to match.

[ExE-Boss]

Like we have unique symbol, we should probably support:

const foo: global symbol "<name>";

This would allow declaring Node’s built-in symbols as:

declare module "util" {
	namespace inspect {
		// https://github.com/nodejs/node/issues/20821
		const custom: global symbol "nodejs.util.inspect.custom";
	}

	namespace promisify {
		// https://github.com/nodejs/node/issues/31647
		const custom: global symbol "nodejs.util.promisify.custom";
	}
}