Bad error message for assigning empty array to tuple type

[Arnavion]

Reported in the IRC channel against 1.7.5. Still exists in 1.8.0-dev.20160117

var x: [number] = [];

foo.ts(1,5): error TS2322: Type 'undefined[]' is not assignable to type '[number]'.
  Property '0' is missing in type 'undefined[]'.

I guess undefined comes from the fact that the RHS array is empty, so its element type is left uninitialized. Maybe you want to detect that and have a different error message? "An empty array is not assignable to type [number]." or something.

[RyanCavanaugh]

@sandersn @DanielRosenwasser would this be fixed by the proposed contextual typing changes?

[DanielRosenwasser]

If I understand things correctly, you should get number[] with Nathan's changes.

Even if you didn't, I think I've concluded on several occasions with @JsonFreeman and @CyrusNajmabadi that surfacing the undefined type in error messages is not a problem.

[Arnavion]

Even if you didn't, I think I've concluded on several occasions with @JsonFreeman and @CyrusNajmabadi that surfacing the undefined type in error messages is not a problem.

You don't think it gives the impression of being a compiler bug?

[DanielRosenwasser]

That's what I argued at the time because it totally looks like we screwed up somehow. The fact that we're surfacing the undefined type isn't a problem so much as it is that its representation kinda, well, sucks. 😄

[sandersn]

I checked, looks like the error with the change in #6118 is still the same.

[JsonFreeman]

Is there such thing as a tuple of length 0? It seems like the error ought to say something like

type [] is not assignable to type [number]

where [] is the tuple of length 0. But I guess a tuple of length 0 is really the same as undefined[] anyway, so maybe there is no point in having that concept.

This is not to say that the type undefined should never be surfaced in an error message. That is a separate discussion. I think the better question is not about undefined, but about how to improve error messages about tuple types. I think that while technically, a tuple type acts as an object with numeric properties, perhaps the error should talk about the "size / length" of the tuple. I think that would be more intuitive for users.

[DanielRosenwasser]

Is this still a bug given that the null and undefined types are going to be representable in 2.0?

[Arnavion]

(1.9.0-dev.20160402)

foo.ts:

var x = [];

tsc -d -t es5 foo.ts

foo.d.ts:

declare var x: any[];

---

foo.ts:

var x: [number] = [];

tsc -t es5 foo.ts

foo.ts(1,5): error TS2322: Type 'undefined[]' is not assignable to type '[number]'.
  Property '0' is missing in type 'undefined[]'.

There's no difference with --strictNullChecks

---

So is an empty array any[] or undefined[] ? Should it be undefined[] even if --strictNullChecks isn't passed in?

[JsonFreeman]

For the tuple case, it doesn't really matter what the element type is. The array literal is required to have an element at index 0 because of the contextual type, but it does not.

For the other case, if the array did not get widened to any[], I don't think you would be able to subsequently add elements to it, which would be very limiting.

Both of these ideas are independent of strict null checking. That said, I still think it would be reasonable to have tuples of length 0, and then the message would talk about the type [] as the empty tuple type.