Acces of object with index signature via key with generic type returns wrong type

[pablobirukov]

TypeScript Version: [email protected], [email protected]

Code

declare const dict: { [K: string]: number | undefined };
function foo<S extends string>(betSpot: S): number {
    const value = dict[betSpot];
    return value ? value : 0;
//  ~~~~~~~~~~~~~~~~~~~~~~~~~
}

Expected behavior:

No error

Actual behavior:

Type '{ [K: string]: number | undefined; }[S]' is not assignable to type 'number'.
  Type 'number | undefined' is not assignable to type 'number'.
    Type 'undefined' is not assignable to type 'number'.

Playground Link: http://www.typescriptlang.org/play/index.html#src=declare%20const%20dict%3A%20%7B%20%5BK%3A%20string%5D%3A%20number%20%7C%20undefined%20%7D%3B%0D%0Afunction%20foo%3CS%20extends%20string%3E(betSpot%3A%20S)%3A%20number%20%7B%0D%0A%20%20%20%20const%20value%20%3D%20dict%5BbetSpot%5D%3B%0D%0A%20%20%20%20return%20value%20%3F%20value%20%3A%200%3B%0D%0A%7D

Related Issues:

#21770 #21760 seem related

[mhegazy]

looks like a duplicate of #22214

[mhegazy]

dict[betSpot] is considered to be generic by the compiler, and thus not narrowed. but it does not have to be since dict has a string indexer.

[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.