“T[][] is not assignable to Iterable<[T, T]>” while it is (at compile time)

[msklvsk]

TypeScript Version:

1.8.9

Code

new Map<number, number>([1, 2].map(x => [x, x]));

// Error:
// Argument of type 'number[][]' is not assignable to parameter of type 'Iterable<[number, number]>'…

Expected behavior: types are inferred correctly, no error shown.
Actual behavior: type error.

[FranklinWhale]

I think this issue is related to #6574

[msklvsk]

Thanks, yes, same problem, a dupe. Should I close this one?

[Arnavion]

It would be nice if contextual types allowed to infer an array as a tuple, to support such an idiom.

Franklin Tse (@FranklinWhale) I don't think #6574 is related. If #6574 was fixed, the inferred type would be Iterable<number[]> which would still be a problem. This issue is about inferring [x, x] as a [number, number] instead of number[].

[FranklinWhale]

I don't mean that this issue is a duplicate of #6574, but they are both related to array to tuple inference. I am sorry if any confusion has been caused.

To be clear:
In #6474, T[] may be a tuple of T.
In this issue, T[][] may be an array of tuples of T.

[Arnavion]

#6574 is about inferring Tuple<T>.map<U>()'s result as a Tuple<U> instead of Array<U>. It says nothing about inferring array literals as tuples (notice that it starts with explicit annotation for the tuple), whereas this issue is about inferring array literals as tuples. I.e., if [x, x] was inferred as a [number, number] instead of number[] then the code in the OP would compile.

Hence my point that they're not related.

[FranklinWhale]

That's not really the case. In Eli Barzilay (@elbarzil)'s code sample in #6574, [1,2] in [1,2].map(x=>x+1) is actually a number[], and the map function also returns a number[]. It is actually the same as assigning an array created from an array literal to a varible declared as a tuple. Therefore, #6574 will be fixed when any array may be inferred as tuple when there is an matching tuple declaration.

[FranklinWhale]

See also #3369