no-useless-assertion: false positive when disabling no-misused-generics

[AlCalzone]

Consider the following code:

/* wotan-disable no-misused-generics */
function foo<T = unknown>(): T {
	return ({} as any) as T;
}

const bar = foo() as string;
console.log(bar.length);

wotan complains about the as string being a useless assertion. However when I remove it, the type of bar changes to unknown, making the last line illegal.

[ajafff]

I remember seeing this in the TSLint repo. This was discussed in palantir/tslint#3540 and microsoft/TypeScript#20681

TL;DR: this not only occurs when disabling no-misused-generics, but every time a call(like) expression has no inference candidate for a type parameter used in the return type. Instead of using the declared fallback the type is inferred from the contextual type. In this case the contextual type comes from the type assertion. If you remove the assertion, there's no contextual type and therefore the type parameter default applies.

You can work around this design limitation by providing the contextual type by any other means:

const bar: string = foo(); // 'T' is inferred as 'string'

---

I don't think this can be fixed in this project. If you care enough about this issue, please consider opening a new issue with TypeScript.

[AlCalzone]

If you care enough about this issue

I don't, since I can just pass the intended type in the angled brackets. I just thought the behavior is weird.