Repository navigation
Inference of [a,b] to union type array is missing index signature [0]:A and [1]:B #9216
Description
Activity
it's a well known annoying problem with tuples among many others,
it doesn't work the way you expect because array literals are inferred being array-types not tuple-typesa workaround i use:
function of2<a, b>(one: a, two: b) : [a, b] { return [one, two]; } ... var promises = of2(a, b); $q.all(promises).then(/*.. here comes tuple ..*/);As noted by Aleksey-Bykov, the compiler can not infer any array literal as a tuple, as this would be a massive breaking change. so by the time we get to the call,
promisesis just an array or variable length.Could it infer type to
(A|B)[] & [A,B]which should not be a breaking change? Or have I missed something?Seems to be effective:
const a = 1; const b = 'string'; const p: (number | string)[] & [ number, string ] = [a, b]; function all<T1, T2>(values: [T1, T2]): [T1, T2] { return; } function some<T>(values: T[]): T { return; } const r1 = all(p); const r2 = some(p);
If you inferred that, you wouldn't be able to assign a
stringtop[0], which(number | string)[]does allow.- locked and limited conversation to collaborators
on Jun 19, 2018
When calling a method declared as
all<T1, T2>(values: [T1, T2]);I cannot use a union type array.TypeScript Version:
1.8.10
Code
OK:
Error:
Expected behavior:
It would be nice if both versions work, without explicitly reference types A and B. I think the problem is, that
[a, b]infers to(A|B)[]instead of[A,B], which should prevent me from[a,b].push(new C())(Is this the intension?). The problem is, that(A|B)[]looses the indexers[0]: Aand[1]: B. I think that's what the error message wants to say?Would it be possible to infer
[a, b]to something like(A|B)[A, B]which is compatible toBtw. the error message is rather confusing, since
(A|B)[]actually has an index signature.