Undesirable type argument inference from return position

[DanielRosenwasser]

let x: number[] = Array.from(
    [true, true, false]
)

Today, this code type checks because Array.from<T, U> takes an optional mapfn of type (el: T, index: number) => U.

When U is not inferred from the mapfn, it is instead inferred from the contextual type.

[DanielRosenwasser]

Looks like this is a potential duplicate of #19682 (and was fixed in the signature of Array.from ), but isn't this indicative of a broader problem?

[sandersn]

Yes:

declare function f<T, U = T>(xs: T[], g?: (x: T) => U): U[];
const ns: number[] = f([true]) //should error here: 'boolean' is not assignable to 'number'

The rules for defaults may specify that inference works this way currently, but I believe we decided change those rules in a recent design meeting.

[ajafff]

This was reported a few times as a bug in one of TSLint's rules lately. Ref: palantir/tslint#3540 and duplicates

Would be awesome if this get's fixed.

[DanielRosenwasser]

Spoke to @RyanCavanaugh, seems reasonable to say that this should really be two overloads, but it is still pretty undesirable.

[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.