Bug Report

I came across this odd behavior while doing Day 5 of "Type | Treat".

🔎 Search Terms

as mapped function parameter infer

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Type System Behavior.

I tried every Playground version with as support.

⏯ Playground Link

Playground link with relevant code

💻 Code

function bug<Es extends 'foo' | 'bar'>(_: { [E in Es as `on${E}`]: (e: string) => void }) {
}

bug({
  onfoo(_e) {}, // _e: any; onfoo: (e: string) => void
  onbar(_e) {}  // _e: any; onbar: (e: string) => void
})

function workaround_1<Es extends 'foo' | 'bar'>(_: { [E in `on${Es}`]: (e: string) => void }) {
}

workaround_1({
  onfoo(_e) {}, // _e: string
  onbar(_e) {}  // _e: string
})

function workaround_2<Es extends 'foo' | 'bar'>(_: Es extends never ? {} : { [E in Es as `on${E}`]: (e: string) => void }) {
}

workaround_2({
  onfoo(_e) {}, // _e: string
  onbar(_e) {}  // _e: string
})

🙁 Actual behavior

The types of onfoo and onbar are correctly inferred as (e: string) => void but the parameter _e is incorrectly typed as any instead of string.

🙂 Expected behavior

I would expect _e to have type string, because it's known that the function has type (e: string) => void. I provided two variations that are typed correctly -- one without using as, and one forcing the type to be conditional on Es. All three should be equivalent. I'm especially surprised that bug and workaround_2 behave differently since it's (presumably) just forcing Es to be evaluated slightly earlier.