`asserts this is T` does not narrow type of `this`

🔎 Search Terms

• asserts this is T
• narrow
• assertions
• I also checked #59707, but it is unrelated (generic types).

🕗 Version & Regression Information

• Playground: v5.7.0-dev.20241004
• At least since v5.6.2

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYGwhgzhAECC0G8BQ1oHsB2ICeARApgGYCWG+AJgJIawAUAlIiqtMJhGiPgHQhoDmtAESwh9ANzMAvkmaQI+AE4AXOvQBc0eUuUxlAC2IwjcRNBkzZoedABCTVNpVrNT3dAMmT8BOaSXlbAAHfGgAYWgAXlMAHztJJEIAVwxgZWJMLSgdOmBNMI0shRUYYGhvJktk1PTM5XwIZQBGWjzwxmRUYG43NUku7kwcAhIyKhoGcWgAemnoAFFFRTRFTQAFZZCVbGgAciG8IlIKalhd6HI0BugMNGVofAAPI3u64NDdsN3uaA20LcCewOI2O4zOFyuMFu9yeL3QGA87z2tm+-lk1TSGQR9UaACZWvkOnJss5WhJmN1gUcxqdJjM5gB1FYAawgAEI0dAgA

💻 Code

class A {
  onlyDefinedInA() {
    console.log("A");
  }

  assertA(): asserts this is A { }
}


class B {
  assertA(): asserts this is A { }
}

type C = A | B;

function assertA(c: C): asserts c is A {
}

function test1(c: C) {
  c.assertA();
  c.onlyDefinedInA(); // Error: Property 'onlyDefinedInA' does not exist on type 'C'. Property 'onlyDefinedInA' does not exist on type 'B'.
}

function test2(c: C) {
  assertA(c);
  c.onlyDefinedInA(); // Works!
}

Workbench Repro

🙁 Actual behavior

Type of c is not narrowed after the method call, producing an error.

🙂 Expected behavior

I expect the member function to to perform type narrowing.

Additional information about the issue

No response

Commencez par la reproduction TypeScript Playground liée et comparez le cas d’appel de méthode dans test1 avec l’assertion autonome dans test2. Suivez la façon dont asserts this is A est traité après un appel de membre, puis vérifiez que l’appel de méthode affine c de sorte que onlyDefinedInA() soit accepté sans provoquer de régression dans le cas autonome.