Poor typing for array iterator

[pauldraper]

TypeScript Version: 3.6.3

Search Terms: Array iterator

Code

const array = [1];
const iterator = array[Symbol.iterator]();
const value: string = iterator.next().value; // compile error

Expected behavior:

A type check error that number cannot be string.

Actual behavior:

No type check error.

Playground Link: https://www.typescriptlang.org/play/?ssl=1&ssc=1&pln=4&pc=1#code/MYewdgzgLgBAhgJwXAnjAvDA2gRgLoDcAsAFCiSwCWUApslCAhvEqlgMooC2ARiADYA6anTgMEeABQBKYmXDQYANzj8ArjQBcMaAkpgA5sxH1GgsDQAeUGYJXqaBGAHpnMUFwAOlfjRh0ERlIgA

Related Issues:

I do not that Iterators/Generators were changed recently. IDK if that is related.

[AviVahl]

You'll get correct type error if you verify that !result.done:

const array = [1];
const iterator = array[Symbol.iterator]();
const res = iterator.next()
if (!res.done) {
    const num: string = res.value
}

[fatcerberus]

Cause of this is that IterableIterator<number> is an alias for Iterator<number, any, undefined> (thanks to the changes to iterator typings in 3.6) so without checking the value of done first, value is typed as number | any, which collapses to just any.

[AviVahl]

I wonder whether it would have been possible to use unknown as default for the iterator's TReturn type..? @rbuckton might be able to clarify.

[AviVahl]

Oh, see discussion at #33353

[rbuckton]

This is essentially a duplicate of #33353.