Bad type inference under --strictFunctionTypes

[gcnew]

TypeScript Version: 2.7.0-dev.20171029

Code

type Comparator<T> = (x: T, y: T) => number;

interface LinkedList<T> {
    comparator: Comparator<T>,
    nodes: Node<T>
}

type Node<T> = { value: T, next: Node<T> } | null

declare function compareNumbers(x: number, y: number): number;
declare function mkList<T>(items: T[], comparator: Comparator<T>): LinkedList<T>;

const list: LinkedList<number> = mkList([], compareNumbers); // `T` inferred as `never`,
                                                             //     should   be `number`

Actual behavior:

The implicit evolving never[] type of the array literal is taking precedence, leading to unexpected results.

[HerringtonDarkholme]

I think this is an intended behavior. compareNumbers type argument is covariant. This means number must be a supertype of T. Empty array is typed initially as never, a subtype of every other type. So, type checker should choose never, so number is correctly typed covariantly.

I would suggest annotate mkList<number>

[kitsonk]

That does feel like never is infecting the inference though. While it might be strictly sound it feels like resolution should only assign never when there is no other suitable type.

[gcnew]

I think the main problem is that an unresolved contextually typed literal takes precedence. The array is not a variable reference, it's a literal. What surfaces is the presumably evolving implicit never[] type, which is unexpected because it trumps user provided annotations.

[mhegazy]

One option here is to make no inferences form a fresh array literal. the parallel in an object literal case here would be:

declare function mkList<T>(items: { 0?: T }, comparator: Comparator<T>): LinkedList<T>;
mkList({}, compareNumbers); // `number`, since no inferences are made from `{}`

[gcnew]

Yes, I think that would be the best approach, but only if the array is empty.

[RyanCavanaugh]

Example of breaking soundness with an aliased never[]

type Comparator<T> = (x: T, y: T) => number;

declare const compareStrings: Comparator<string>;
declare const compareNumber: Comparator<number>;

function makeArray<T1, T2>(arr1: T1[], arr2: T2[], c1: Comparator<T1>, c2: Comparator<T2>) {
    return {
        arr1,
        arr2
    }
}

const arr: never[] = [];
const res = makeArray(arr, arr, compareStrings, compareNumber);

res.arr1.push("");
res.arr2.push(10);