(Set | Array).forEach loses type inference for callback

[iansan5653]

Code

type ListOfNumbers = Set<number> | number[];

function iterates(list: ListOfNumbers) {
  list.forEach(element => {
    element; // has type any
  });
}

Expected behavior:

The type of element should be number, because the type of forEach should be:

type ForEachSetOrArray<T> = (current: T, index: T | number, list: Set<T> | Array<T>) => void;

Note: The type of index is not a mistake - Set gives the current value twice, while Array gives the current index. It's weird.

Actual behavior:

The type of element is inferred to be any and has to be explicitly stated as number.

Playground Link: https://www.typescriptlang.org/play/?ssl=1&ssc=9&pln=1&pc=1#code/C4TwDgpgBAMglgZ2AeQGYDkCuBbARhAJwSgF4oBlCYAHgDsd8CA+KAHynr0IG0BdAbgCwAKBGpMtAMbA4Ae1pQ4wQgENlCABQAbRMABcsXWixciASigBvEVCg6kAOlSyCAURWSAFhohaI2CFpgUhZrYVtbX39A4H4oAHp4qE8VYlBIKBVaEBsoAF8zIWE8oA

[RyanCavanaugh]

We're only capable of combining contextual types when the source signatures come from the same declaration; making the parameters line up in the first position wouldn't be sufficient here.