union of function types should be narrowed by their return type

[DetachHead]

Bug Report

🔎 Search Terms

narrow function types by return type

🕗 Version & Regression Information

4.8.0-dev.20220719

⏯ Playground Link

Playground link with relevant code

💻 Code

declare let f1: (() => number) | (() => string)
declare let f2: () => (number | string)

if (typeof f1() === 'string') {
    const foo: string = f1() //should be narrowed to string but isn't
}

if (typeof f2() === 'string') {
    const foo: string = f2() //correctly not narrowed
}

f1 = f2 //correct error

🙁 Actual behavior

f1 is not narrowed to () => string

typescript does seem to recognize that the two types () => (number | string) and (() => number) | (() => string)
are different though, as it does not allow assigning f2 to f1

🙂 Expected behavior

f1 is narrowed to () => string, as the union type (() => number) | (() => string) means that the function either only ever returns a string or only ever returns a number. so if the first function call returns a string it's safe to say that subsequent calls will also return a string

[RyanCavanaugh]

Makes sense. I'm curious where this comes up.