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Inference of [a,b] to union type array is missing index signature [0]:A and [1]:B #9216
Description
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:
$q.all([a, b]).then(/*...*/);Error:
var promises = [a, b];
$q.all(promises).then(/*...*/);
> TS2329 Index signature is missing in type '(IPromise<A> | IPromise<B>)[]'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]: A and [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 to
interface IAB {
[i:number]: A|B;
[0]:A;
[1]:B;
}Btw. the error message is rather confusing, since (A|B)[] actually has an index signature.
Activity
zpdDG4gta8XKpMCd commentedon Jun 16, 2016
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-types
a workaround i use:
mhegazy commentedon Jun 16, 2016
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.iskiselev commentedon Jun 17, 2016
Could it infer type to
(A|B)[] & [A,B]which should not be a breaking change? Or have I missed something?kitsonk commentedon Jun 17, 2016
Seems to be effective:
DanielRosenwasser commentedon Jun 18, 2016
If you inferred that, you wouldn't be able to assign a
stringtop[0], which(number | string)[]does allow.