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

💻 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

According to the source code, it's deliberate (link):

// Constituent type predicates must all have matching kinds. We don't create composite type predicates for assertions.

If it's not supported it would be nice if it produced an error similar to #33622 instead of just silently failing.

Ryan Cavanaugh (@RyanCavanaugh)

Ryan Cavanaugh (@RyanCavanaugh) could you clarify what is the bug here? should it work or should it error in a visible way like Joe Calzaretta (@jcalz) is suggesting? I think the latter but it would be great to have confirmation

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Unions of assertion functions do not act as assertion functions #59707

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Unions of assertion functions do not act as assertion functions #59707

Description

🔎 Search Terms

🕗 Version & Regression Information

⏯ Playground Link

💻 Code

🙁 Actual behavior

🙂 Expected behavior

Additional information about the issue

Activity

Andarist commented on Aug 21, 2024

jcalz commented on Aug 21, 2024

Andarist commented on Aug 21, 2024

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