Get union of all possible return types of a function

[bcherny]

For example:

type F = () => number | string
type G = returnof F // number | string

If there is interest, I will write up a proposal.

Note that this is distinct from #6606, which gives a return type given parameter types.

[mhegazy]

I think the example should be

type F = {
    (): number;
    (): string;
}

[bcherny]

You're right - your example shows more clearly that we're taking the union of all return types. In this example, returnof both my F and yours should give number | string.

[bcherny]

This is now done with ReturnType in #21847.