User-defined type guard method can be overridden with any non-type-guard return type

[mattmccutchen]

TypeScript Version: master (07966dc)

Search Terms: user-defined type guard predicate method override overridden return

Code

interface A {
    isB(): this is B;
}
interface B extends A {
    isB(): "this is nonsense";
}

Expected behavior: Error on "this is nonsense".

Actual behavior: No error.

Playground Link: link

Related Issues: none found

[mattmccutchen]

@RyanCavanaugh I'll be happy to make a PR if you tell me the intended behavior. A this type guard function can be overridden with any function returning a boolean, or it can only be overridden with a this type guard function for the same type (as is the rule for parameter type guards and is sound)? The previous discussion I found is here.

[RyanCavanaugh]

I'd be curious to see the results of both, but if I had to pick one without seeing RWC results I'd say "must be boolean or the same guarded type".

[gregolai]

I wonder if this issue is related, where a derived method (abstract/non-abstract) does not infer the correct type/interface:

Playground Link: link

There is some type checking going on. For example, if you return a number in the derived method, it correctly shows an error.