Const numbers that index into tuples do not produce the correct type

[sebdoucet]

Hi,

Constants aren't evaluate to access and cast properly union types values:

const ID = 0;
const NAME = 1;

let data: [number, string] = [0, "bob"];

// This need cast to compile
let id: number = data[ID];
let name: string = data[NAME];

Is there a reason why it's not supported ?

Regards,
Sébastien

[RyanCavanaugh]

We did this for const enums (#3411); I don't see why we wouldn't for this case either

[sebdoucet]

It's not compile because you need a cast, it does not recognize ID as 0, so it does not detect you try to get the first entry of the union types, so it does not cast it.

What is important to read is the tuple have first entry as number and second entry as string. Without const, data[0] return a number and data[1] return a string. With const it does not work.

[RyanCavanaugh]

Accepting PRs.

See also linked issues #6804 and #6805

[DrSammyD]

class x{
   public y(){ return "string" }
}
var z= new x;
const y = "y";
var w = z["y"]();
var v = z[y]();
w.length; //Type safe
v.length; //No longer type safe

Should this be changed too?

[mhegazy]

Should this be changed too?

i do not see why not.

[zpdDG4gta8XKpMCd]

got bitten by it, please fix asap:

nightly build Oct 24, 2016

const values: [number, string] = [1, 'a'];

const one = values[0]; // number, OK

const index = 0; // index is `0` (not just a number)
const another = values[index]; // expected number, actual: string | number <-- WHY?

[JoshuaKGoldberg]

@RyanCavanaugh this seems to be fixed when checking TypeScript 3.3.3. id and name from the OP are correctly number and string with and without their explicit type annotations.