Auto-array typing not working in 4.4

[DanielRosenwasser]

function unique(elems: string[]) {
    const arr = [];
    for (const e of elems) {
        if (!arr.some(x => x === e)) {
            arr.push(e);
        }
    }
    return arr;
}

Expected: The type of arr is string[].
Actual: An error under noImplicitAny.

Variable 'arr' implicitly has type 'any[]' in some locations where its type cannot be determined.

Playground Link

Originally found in #45117, reported by @mkubilayk.

[Kingwl]

Possible related with #40359 ?

[ahejlsberg]

This is working as intended. We complain about arr having an implicit any[] type because it is possible for control to reach the arr.some(...) call before any elements have been added to the array. It's equivalent to what happens in this example:

const a = [];
if (!a.some(x => x === 'foo')) {  // Error, a implicitly has type any[]
    a.push('bar');
}

Now, you could argue that the implicit any types don't matter because when the array is empty the arrow function is never called. But that's only because we know the semantics of the some method. It's not a general assumption we can make.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.