Tuples constructed via array literals don't typecheck against tuple constraints

[ccapndave]

type Point = [number, number]; var x = [1, 2]; var p: Point = x;

In this case p won't typecheck because x has been inferred as number[] instead of [number, number]. It would be really nice if the type system could check against both types.

[mhegazy]

Tuple types are not inferred from arrays, they have to be specified explicitly. so in your example:

var x: [number, number] = [1, 2];

or the type alias way as you specified.

Changing how an array literal type being inferred would be a huge breaking change that would not be feasible at this point; also it is not clear if this would be more helpful than inferring an array in the general case.

[Arnavion]

@mhegazy Since tuple types decay to the corresponding array type, will it really be a breaking change?

[DanielRosenwasser]

@Arnavion what about the following case?

var x = [1, 2];
x = [1]

[mhegazy]

tuples are more restrictive than arrays, since they have a size constraint. just like in @DanielRosenwasser example.

[Arnavion]

Ah, right.

[ccapndave]

I suppose at this point it could fall back to the least restrictive type, but I do see that its going to be pretty complicated (and maybe a bit unpredictable).

[mhegazy]

The problem is there is not really a fall back.. the type is either an array or a tuple.. inferring a tuple would enable some scenarios but would break others and vice versa.

[FranklinWhale]

If the length of an array is known and its type matches with a tuple declaration, should the assignment be allowed?

See also #6574 and #7799