Type Guards are no longer working correctly all the time

[MaksiRose]

Bug Report

🔎 Search Terms

Type Guard

🕗 Version & Regression Information

• This changed between versions 4.9.5 and 5.0.4

⏯ Playground Link

A simple example

💻 Code

declare class Foo<Stuff extends boolean> {
  stuff: Stuff extends true ? string : string | undefined;
  isReady(): this is Foo<true> 
}
declare const x: Foo<boolean>;
if(x.isReady()) {
  x
//^? - const x: Foo<boolean>
// Should be Foo<true>
}

🙁 Actual behavior

The Type Guard didn't change the type, which is unintuitive behavior.

🙂 Expected behavior

The method should act as a type guard and change the type

[Andarist]

Might be related to #53436

[graphemecluster]

The same bug happens in the code snippet from #9619 (comment) and it still presents in the latest nightly build.
Could anyone (perhaps @RyanCavanaugh?) please bisect it?

[Andarist]

@graphemecluster your issue (and this one here) is caused by: #52984 , the one that I linked to here (#53436) was broken before that though

[RyanCavanaugh]

Foo<true> isn't considered a strict subtype of Foo<boolean> due to the variance measurement coming out too conservative when looking at stuff. TS doesn't have smarts to identify that a conditional type with T in the true branch and T | U in the false branch is actually covariant on the tested type parameter - I'm not entirely sure how feasible it would be to do that but it would at least be possible.

Counterexample that works:

declare class Foo<Stuff extends boolean> {
  observ: Stuff;
  isReady(): this is Foo<true>; 
}

declare const x: Foo<boolean>;
if (x.isReady()) {
  x;
  //^? - const x: Foo<true>
}

In this case you can write a variance annotation to make it work as desired:

declare class Foo<out Stuff extends boolean> {

[graphemecluster]

@RyanCavanaugh Understandable, but #52984 is still a breaking change.