`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

Start with the linked TypeScript Playground repro and compare the method-call case in test1 with the standalone assertion in test2. Trace how asserts this is A is handled after a member call, then verify that the method call narrows c so onlyDefinedInA() is accepted without regressing the standalone case.