Assignability to asserts type predicate is not checked

[gabritto]

🔎 Search Terms

asserts assignability

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.5.0-dev.20240416&ssl=9&ssc=2&pln=1&pc=1#code/JYOwLgpgTgZghgYwgAgGIHt3IN4ChkHJwDOx0YAFAB4BcyAriANYjoDuIAlHSWVGMWRVkwQQCI4YgNz5CvcgCE4UanUYt2XHqXKCwAC1EjBSqDIC+uXKEixEKU8ghVIIACaCMWPISI7+qgzMrBzcyCD0ALYARtAyvvL8poHqIVrIxGBQoADmFkA

💻 Code

interface Foo {
    assert(x: unknown): asserts x is "a";
    assertBar(x: unknown): asserts this is Bar;
}

interface Bar extends Foo {
    assert(x: unknown): number;
    assertBar(x: unknown): string;
}

🙁 Actual behavior

No errors on Bar.

🙂 Expected behavior

Errors on Bar about assert and assertBar's signatures not being assignable to their counterparts in Foo.

Additional information about the issue

Follow up of #57103.

[fatcerberus]

Simplified repro:

declare type Asserter = (x: unknown) => asserts x is "a";
declare const fun: (x: unknown) => number;
const assert: Asserter = fun;  // no error

It has nothing to do with assignability checking during inheritance - it's that non-assertion functions are, in fact, (unsoundly) assignable to assertion function types. This is probably a side effect of (x: unknown) => number being assignable to (x: unknown) => void.

[mwoenker]

I think this is effectively a duplicate of this issue #49262