Unions of assertion functions do not act as assertion functions

🔎 Search Terms

narrowing, assertion function

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about everything

⏯ Playground Link

https://tsplay.dev/Wv3DYw

💻 Code

  class box<T> {
    constructor(public value: T){}

    check(): this is box<string> {
      return typeof this.value == 'string';
    }

    assert(): asserts this is box<string> {
      if (typeof this.value != 'string') throw new Error();
    }

    private test() {
      this.assert();
      // type correctly narrowed
      this.value.substring(0);
    }
  }

  function make() : box<string> | box<number> {
    return new box('a');
  }

  function assert(b: box<string> | box<number>): asserts b is box<string> {
    if (typeof b.value != 'string') throw new Error();
  }

  const b = make();

  if (b.check()) {
    // type correctly narrowed
    b.value.substring(0);
  }

  b.assert();

  // type not narrowed (substring does not exist on type 'string | number')
  b.value.substring(0);
   
  assert(b);

  // type correctly narrowed
  b.value.substring(0);

🙁 Actual behavior

Type is not narrowed after method call

🙂 Expected behavior

I expected the type to be narrowed after method call since it is narrowed within other methods or when using a function external to the type.

Additional information about the issue

https://stackoverflow.com/questions/78879014/typescript-type-assertion-does-not-narrow-class-instance-from-union

Start with the linked TypeScript Playground reproduction and compare the narrowing produced by b.check(), b.assert(), and the external assert(b) call. Investigate why the method call does not narrow the union, then verify that the method assertion behaves consistently with the external assertion while preserving the existing successful cases.