Warning when writing a generator function without any yield

[krryan]

TypeScript Version: 2.2.0-dev.20170126

Code

export function* keyed<K, V>(values: Iterable<V>, toKey: (value: V) => K): IterableIterator<[K, V]> {
    for (const value of values) {
        return [toKey(value), value];
    }
}

Expected behavior:
Some kind of notice that this generator function never actually yields anything.

Actual behavior:
Compiles, and runs for that matter, without error, just silently yielding nothing.

I just spent a couple of hours tracking down a bug here, where return was accidentally used instead of yield. I'll admit I'm only starting to learn about generator functions and their uses, so it may be entirely possible that there are generator functions that want to return, rather than yield, in some cases, but my understanding is that every generator function should yield at some point.

Could we get a compiler warning for something like this? I'm not looking for a halt detector or anything :P just some kind of warning like "No 'yield' in reachable code," or something.

[RyanCavanaugh]

Accepting PRs for the rule: a generator function with a return statement must not have zero yield return statements (please make the error message less spec-like!)

[mhegazy]

#14043 has a valid example, a default implementation for a generator. the alternative would be to implement a function that returns the generator protocol, but that seems overly verbose.

[mhegazy]

In light of the discussion about errors vs warnings we should instead move this to tslint.

[rbuckton]

Note: #30790 will address this only in cases where the return type is explicitly specified as Generator<T, void>, as returning the value above would not be assignable to void.