Typescript: `K extends keyof ...` not being narrowed in a switch case

Created on 3 Jun 2019 · 2Comments · Source: microsoft/TypeScript

TypeScript Version: 3.5.1

Search Terms:
extends keyof narrowing
infer not narrowing type

Code

type MethodArguments<T> = [T] extends [(...args: infer U) => any]
  ? U
  : [T] extends [void] ? [] : [T];

interface User {
  setName(name: string): void;
  setAge(age: number): void;
  setBirthday(year: number, month: number, day: number): void;
  getName(): string;
  getAge(): number;
  getBirthday(): { year: number, month: number, day: number };
}

class SomeClass {
  constructor(public user: User) { }

  handle<K extends keyof User>(method: K, ...args: MethodArguments<User[K]>): void {
    let result: ReturnType<User[K]>;

    switch (method) {
      case "setName":
        const [
          name
        ] = args;

        result = this.user.setName(name);

        break;
      case "setAge":
        const [
          age
        ] = args;

        result = this.user.setAge(age);

        break;
      case "setBirthday":
        const [
          year,
          month,
          day
        ] = args;

        result = this.user.setBirthday(year, month, day);

        break;
      case "getName":
        result = this.user.getName();

        break;
      case "getAge":
        result = this.user.getAge();

        break;
      case "getBirthday":
        result = this.user.getBirthday();

        break;
    }

    console.log(result);
  }
}

Expected behavior:
In the switch case each case should narrow the type of method, args and result. However, this doesn't happen.

Actual behavior:
What's actually occurring is that in each switch case, method has the type K extends "setName" | "setAge" | "setBirthday" | "getName" | "getAge" | "getBirthday". Its type is not being narrowed to one of the keys of User. This problem also persist for the args and result variables. They're not being narrowed to their respective types.

Duplicate

Source