Incorrect return type for Array.prototype.reduce()

[kwpeters]

The TypeScript compiler thinks the type of the instances.reduce() call below is C1 | string. I believe the return type should be string.

Code

class C1
{
}

type ItemType = C1 | string;

const instances: Array<ItemType> = [new C1(), "foo", new C1(), "bar"];

// The following line produces
// error TS2322 Type 'ItemType' is not assignable to type 'string'.
// Type 'C1' is not assignable to type 'string'.
const result: string = instances.reduce(
    (acc: string, curItem: ItemType): string => {
        if (curItem instanceof C1) {
            return acc + "C1";
        } else {
            return acc +curItem;
        }
    },
    ""
);

Expected behavior:
I would expect the above code to compile without errors. If I use the Lodash's version of reduce() (_.reduce()) it compiles successfully.

Actual behavior:
The assignment of the result variable results in "error TS2322 Type 'ItemType' is not assignable to type 'string'. Type 'C1' is not assignable to type 'string'." When evaluating the type of the instances.reduce() expression, it is C1 | string. I would expect it to be just string.

[MrAndersen1]

Since acc (string) is assignable to curItem (C1 | string) typescript cannot tell which overload of reduce you want to use. You can specify this with:
const result: string = instances.reduce<string>( ...
Hope that makes sense. Why curItem is "never" in the "else" is curious to me however...

[kwpeters]

Thanks for your help. Using instances.reduce<string>(...) does fix the problem.

I do, however, have some questions about your statements above:

1. curItem should not have type never in the else block. It should have type string, shouldn't it?
2. Why must I specify the return type of reduce() (via the generic parameter) when it is plain to see that the function passed to it can only return a string? I would expect the compiler to be able to infer this. Even so,I have explicitly specified the function's return type, and I still get the same error.

[mhegazy]

this is caused by the overload of Array.prototype.reduce:

   reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T;

since string is a subtype of ItemType the inferences for string and ItemType are reduced to ItemType, and that overload is picked. i am not sure why we have this overload though.

I need to remove it and see what breaks :)

[yortus]

Duplicate of #7014?

[mhegazy]

thanks @yortus

[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.