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Tuples constructed via array literals don't typecheck against tuple constraints #3369
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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.
Mohamed Hegazy (@mhegazy) Since tuple types decay to the corresponding array type, will it really be a breaking change?
Arnav Singh (@Arnavion) what about the following case?
var x = [1, 2]; x = [1]
tuples are more restrictive than arrays, since they have a size constraint. just like in Daniel Rosenwasser (@DanielRosenwasser) example.
Ah, right.
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).
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.
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on Jun 19, 2018
In this case
pwon't typecheck because x has been inferred asnumber[]instead of[number, number]. It would be really nice if the type system could check against both types.