Compiler does not narrow tuples based on length property

[eyyubovkamran]

TypeScript Version: 2.7.0-dev.201xxxxx

type A = [string] | [string, number];

let x: A;
if(x.length > 1){
    let y: number = x[1];
}

Expected behavior:
Should compile successfully.

Actual behavior:
error TS2322: Type 'string | number' is not assignable to type 'number'.
Type 'string' is not assignable to type 'number'.

[RyanCavanaugh]

Hijacking this to represent the proposed literal length change proposed at the design meeting today

cc @DanielRosenwasser @ahejlsberg

[mhegazy]

related to #19536 and #19463

[mhegazy]

Using > as a discriminant is something we do not see much value in at the time being. but it should work with === assuming we go with proposal in #17765, which will give tuples an explicit length property with a literal type, and hence will allow length to act as a discriminant property.

[mhegazy]

Should be subsumed by #19463 and #17765.

[sandersn]

Yes, narrowing with === works with #17765.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.