Tuple type not inferred when calling generic function

[andy-hanson]

Tuple types aren't always inferred when calling generic functions. For example:

function identity<A>(value: A): A {
    return value
}

// works
const [a1, b1]: [number, string] = [1, 'a']

// works
const [a2, b2]: [number, string] = identity<[number, string]>([1, 'a'])

// breaks
const [a3, b3]: [number, string] = identity([1, 'a'])

Output:

foo.ts(12,7): error TS2322: Type '(number | string)[]' is not assignable to type '[number, string]'.
  Property '0' is missing in type '(number | string)[]'.

See also #2189, #4136, #6037.

[DanielRosenwasser]

This is because the array expression needs to be contextually typed by a tuple-like type so that we can infer your type argument as a tuple. You've enforced that when you explicitly gave a type argument, but not in the last example.

One way you could achieve what you want is to add a tuple-like constraint for your type parameter:

function identity<A extends [void] | {}>(value: A): A {
    return value
}

[zpdDG4gta8XKpMCd]

that's the most interesting identify function I've seen so far