Bug Report

A tuple type with a rest element doesn't duck-type properly to the similar tuple type, although they are conceptually identical, causing a ts2345 error to show up when trying to use one with the other.

The type I tested was a type defining a non-empty array. It looks like there's a bug because the location of the "non-empty item" in the definition matters for typescript, although there shouldn't be any difference in the kind of arrays that are accepted for each type.

🔎 Search Terms

• tuple
• rest

🕗 Version & Regression Information

Since 4.2 where rest params where allowed a the start of a tuple too

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

⏯ Playground Link

Playground link with relevant code

💻 Code

type FirstItem<T> = [T, ...T[]];
type LastItem<T> = [...T[], T];

const f0: FirstItem<number> = []
const l0: LastItem<number> = []

const f1: FirstItem<number> = [1]
const l1: LastItem<number> = [1]

const f2: FirstItem<number> = [1, 2]
const l2: LastItem<number> = [1, 2]

function foo(x: FirstItem<number>): void {
    // DO NOTHING
}

foo(f2) // OK
foo(l2) // ts2345

const x: number[] = [2, 3, 4]

foo([1, ...x]) // OK
foo([...x, 5]) // ts2345

🙁 Actual behavior

• ts2345 is shown

🙂 Expected behavior

• the two types should be considered equivalent and duck-typeable.