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

No repository file or test is named. Reproduce the Boolean and keyof examples in the linked Playground, then trace the compiler behavior involved in Boolean members and protected overrides. Done means keyof Boolean includes both "toString" and "valueOf", while the shown assignments and override are checked consistently.