missleading undefined[] in diagnostic message

[zpdDG4gta8XKpMCd]

i suppose that the [] literal doesn't infer undefined as a type of its elements, does it?

[DanielRosenwasser]

Actually it does - try out

const x: string = [null];

and you'll see the null type surface itself as well!

Here's what's happening:

1. We've seen an array literal. We try to get the best common type of its elements.
2. There are no elements to infer from. We default to the undefined type. Why? Well typically we'll be widened to any[] if it matters anyway.
3. x has a type annotation. Rather than inferring the widened type of its initializer, we'll just check against string.
4. Compare undefined[] to string and you get this confusing error message.

We have #6524 to track this issue, but since the null and undefined types are actually going to be representable in 2.0, I'm inclined to say that this is not really bad.