Type narrowing for Tuple-Elements

[christyharagan]

TypeScript Version: nightly (2.2.0-dev.20161210)

Code

function test([a, b]: [number, number] | [string, string]) {
  if (typeof a === 'number') {
    b++
  }
}

Expected behavior:
b to be narrowed to number and the resulting code to throw no compiler errors

Actual behavior:
b has type number | string and thus the code throws a compiler error complaining that b++ can only apply to numbers

[mhegazy]

Same underlying issue as #14957

[christyharagan]

Should I close this in favour of #14957 ?

[mhegazy]

I would leave them both open. though both would be addressed by the same change, ppl tend to think of them as two separate scenarios. this way we have a landing point for users searching for similar issues.

[RyanCavanaugh]

This would work if it weren't for the destructuring. Our CFA algorithm isn't able to correlate values from different places like this; this comes up from time to time but doesn't seem to merit the required work at this time.