Boolean["toString"] inconsistency

TypeScript Version: 3.4.0-dev.20190305

Code

const key: keyof Boolean = "toString"; // error - "toString" is not assignable to "valueOf"

const x: {toString(): string} = new Boolean(); // no error
const y: Boolean["toString"] = true.toString; // no error

class Bool extends Boolean { 
     protected toString() { // also no error, but should be
          return "";
     }
}

Expected behavior: keyof Boolean should be "toString" | "valueOf" Playground

[RyanCavanaugh]

you wrong. keyof of any other type

No, I right.

interface Foo { m; }
// Error
const k: keyof Foo = "toString";

Unlike Boolean, Number, String, and Symbol have explicitly-declared toString methods in their corresponding interface, which is why toString is in their keyofs, which is why I proposed adding toString to Boolean’s interface.

[domenic]

Abbreviated summary: we’re using the TypeScript type system to transform calls of the form x.toString() into AppropriateClass.prototype.toString.call(x). Without accurate information about the methods in x’s prototype chain, this is not possible.

In general, anything which is relying on the TypeScript definitions giving an accurate summary of what methods a class has will encounter this problem.