Tuple types get incorrect contextual type

[Taytay]

TypeScript Version:

$ tsc --version
Version 2.0.0

Code

// concat-bug.ts
var a : Array<[number, number]>= [[1, 2]];

// Typescript detects these first two tuples as arrays of numbers (`number[]`) instead of `[number, number]`
a.concat([[3, 4], [5, 6], [7, 8]]);

Expected behavior:
No compile errors. (It works in the 1.8 playground)

Actual behavior:

$ tsc concat-bug.ts
concat-bug.ts(4,10): error TS2345: Argument of type '[number[], number[], [number, number]]' is not assignable to parameter of type '[number, number] | [number, number][]'.
  Type '[number[], number[], [number, number]]' is not assignable to type '[number, number][]'.
    Types of property 'pop' are incompatible.
      Type '() => number[] | [number, number]' is not assignable to type '() => [number, number]'.
        Type 'number[] | [number, number]' is not assignable to type '[number, number]'.
          Type 'number[]' is not assignable to type '[number, number]'.
            Property '0' is missing in type 'number[]'.

Note that the 3rd parameter is correctly detected as a tuple, but the first two are detected as arrays of numbers.

[Taytay]

Thanks @mhegazy

[sandersn]

This is caused by removing one of the overloads of concat (the ugly one, with an additional, constrained type parameter). At the time we thought it wouldn't break anything. I'll try to find the discussion from that time, but almost certainly the right fix is to put the other overload back.

[sandersn]

The break is in #6629 which happened in response to a long discussion in #6594. We didn't see the tuple problem coming during that discussion, though.

[sandersn]

PR is up at #9997 -- I added an explanation of why this fails there.

[Taytay]

TY for the fix!

On Tuesday, August 2, 2016, Nathan Shively-Sanders notifications@github.com
wrote:

Closed #9901 #9901 via
#9997 #9997.

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