Type parameter leaks

[ghost]

TypeScript Version: 2.7.0-dev.20171108

Code

declare function first<T>(array: T[]): T | undefined;
function f(x?: string[]) {
	return first(x);
}
const x = f([]);

Expected behavior:

Type of x is {} | undefined.

Actual behavior:

Type of x is T | undefined.

[mhegazy]

There is an error resolving the call for first. x of type string[] | undefined is not assignable to T[]. this goes through the resolveErrorCall..

[ghost]

Shouldn't we fill in type parameters in resolveErrorCall?

[mhegazy]

it is an error.. what difference does it make really..

[ghost]

Due to return type inference, the erroneous type can occur arbitrarily far from the erroneous call site. A user won't know where to look first -- they'll just see T come out of nowhere, and since this is a common name they might think it refers to a different T. (This actually happened to me...)

[gcnew]

Considering @ts-ignore, this might lead to surprising results. Checking of the leaked type parameter is off.

declare function first<T>(array: T[]): T;
function mkSkolem(x?: string[]) {
    // @ts-ignore
    return undefined! && first(x);
}

const skolemWitness = mkSkolem(undefined);
const val: typeof skolemWitness = 5; // should be an error

[mhegazy]

@ts-ignore leads to surprising results, and should not be used IMHO.

[gcnew]

Sure, @ts-ignore is obviously a red mark. I'm still concerned with the leaked parameter, though. It seems to behave like an implicit any that's not being flagged. If it were a type not assignable to/from anything, the leak would have been less bad.

[mhegazy]

that's not being flagged

It is.. there is an error!

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.

[ghost]

#19620 will fix this. This was only a problem in services because we forgot to always fill in all the type arguments in the error case pickLongestCandidateSignature that only runs if we're in services.

[ghost]

Fixed by #25100