`no-useless-undefined` with `strictNullCheck` and `<T | undefined>`

[castral]

When you have strictNullChecks enabled in tsconfig, you are explicitly required to pass undefined as a value to a function when it is not optional and a generic or the parameter type for that argument is a union with undefined, for example:

function test<T extends object | undefined>(arg: T): void {

}

test({}); // fine
test(undefined); // fine, but eslint complains on rule unicorn/no-useless-undefined
test(); // not fine, compiler throws TS2554: Expected 1 arguments, but got 0

I ran into this issue with the unicorn/no-useless-undefined rule and worked around it by setting checkArguments: false but it seems like an incorrect implementation to me.

[fregante]

I think the rule is still correct in pointing it out, except that you should fix the function that requires 'undefined' as a parameter instead of the call site.

[castral]

That doesn't really make a lot of sense because in this case it was a third party library that was the call-site: RxJS. And the undefined was a valid usage in the union, it could genuinely be an undefined, uninitialized value. So the rule is incorrect because there are valid corner cases where it throws errors.

[fregante]

Reopening to discuss the removal of the rule for TS files because TS should already be able to tell the user whether undefined is useless or required like in this case.

A new rule could then be devised to cover the definition of functions that "accept useless undefined types"