infer keyword does not work in function return type

[scarf005]

Bug Report

• generic type with infer keyword works on its own
• however, it fails to infer when it's used as return type

🔎 Search Terms

generic return type infer const parameter

🕗 Version & Regression Information

• This is the behavior in 5.0.4 and 5.20 nightly, and I reviewed the FAQ for entries about distributed conditional types

⏯ Playground Link

Playground Link

💻 Code

type Arr = readonly any[]

type Head<T extends Arr> = T extends [infer H, ...infer _] ? H : never
const head = <const T extends Arr>(arr: T): Head<T> => arr[0]

type H = Head<[1, 2, 3]> // 1
const h = head([1, 2, 3]) // should be 1 but never
// const head: <readonly [1, 2, 3]>(arr: readonly [1, 2, 3]) => never

🙁 Actual behavior

type of h is never

🙂 Expected behavior

type of h should be 1

[whzx5byb]

A readonly array is not assignable to a writable array. You should add readonly in the condition type.

- type Head<T extends Arr> = T extends [infer H, ...infer _] ? H : never
+ type Head<T extends Arr> = T extends readonly [infer H, ...infer _] ? H : never

[scarf005]

thank you, must've overlooked it.
Now it works well.