Type guard failures with negative condition

[falsandtru]

TypeScript Version: 2.0.0

Code

class A<T> {
  private _: T;
}
class B<T> {
  private _: T;
}
function f() {
  var o: A<void> | B<void> = new B<void>();
  if (!(o instanceof B)) return o; // B<void>, should be A<void>
  o; // B<any>, should be B<boid>
}

Expected behavior:

A type of o narrowed by type guard inside if statement is A<void>.
A type of o narrowed by type guard after if statement is B<void>.

Actual behavior:

A type of o narrowed by type guard inside if statement is A<void>.
A type of o narrowed by type guard after if statement is B<any>.

[yortus]

I think the 'problem' here is that you assigned a new B<void> instance in the declaration of o. TypeScript has accordingly narrowed o to B<void> for the rest of the function body.

To see the behaviour you are expecting, you can add an <any> cast to prevent the initial narrowing:

class A<T> {
  private _: T;
}
class B<T> {
  private _: T;
}
function f() {
  var o: A<void> | B<void> = <any> new B<void>();            // <=====
  if (!(o instanceof B)) return o; // A<void>, should be A<void>
  o; // B<void>, should be B<boid>
}}

[mhegazy]

@falsandtru i would appreciate it if you can limit the discussion to #9859 and avoid creating much noise.

[mhegazy]

This is the same topic discussed in #9859. closing in favor of #9859