Result of F#foo() is not assignable to ReturnType<F["foo"]>

[jtbandes]

Bug Report

🔎 Search Terms

extends interface union ReturnType, not assignable to ReturnType

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about the above search terms

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Foo {
  foo(): "bar" | "baz";
}
function run<F extends Foo>(f: F): ReturnType<F['foo']> {
  return f.foo(); // works without ReturnType, doesn't work with it
}
let bar = run({ foo: () => "bar" as const });
let _: "bar" = bar; // works with ReturnType, doesn't work without it

🙁 Actual behavior

I'm unable to write a type annotation for function run() that allows the return type to be refined based on the specific F being passed in (with the requirement F extends Foo). When I add the ReturnType annotation, the function body has an error. When I remove it, the function return type is inferred based on the interface requirement of "bar" | "baz" rather than the more specific return type of F["foo"].

Writing return f.foo() as ReturnType<F['foo']> also works, but the as cast is undesirable.

🙂 Expected behavior

It should be possible to write a type annotation for run such that run({ foo: () => "bar" as const }) is inferred to return "bar".

[RyanCavanaugh]

See #24277

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.