(Supposedly) non-empty array causes "Source provides no match for required element at position 0 in target.(2345)"

[twiddler]

Bug Report

🔎 Search Terms

map source provides no match nonempty

🕗 Version & Regression Information

This happens in nightly, 5.0.3, 3.3.3 (similar error message) and many more versions available in the playground.

I reviewed the FAQ for entries about Common "Bugs" That Aren't Bugs.

⏯ Playground Link

Playground link with relevant code

💻 Code

type NonEmptyArray<T> = [T, ...T[]]

const a = [1, 2, 3] as const

function fn<T>(a: NonEmptyArray<T>) {
    return a
}

fn(a.map((v) => v)) // Argument of type '(1 | 2 | 3)[]' is not assignable to parameter of type 'NonEmptyArray<1 | 2 | 3>'. Source provides no match for required element at position 0 in target.(2345)

🙁 Actual behavior

TypeScript complains that a.map((v) => v)) is not a valid input for fn as it might be missing an element at position 0.

🙂 Expected behavior

I might be mistaken, but I understand that map guarantees an array of equal length, which in this case would guarantee an element at position 0.

Thanks for your help. 🙏

[jcalz]

Essentially a duplicate of #29841

[twiddler]

I think so, too. Thanks!