concat arrays

[SancheZz]

TypeScript Version: 4.1.2 & Nightly 4.2

Search Terms: concat, spread

Code

type First = [number, string];
type Second = [string, string];
type Result = [...First, ...Second];

function concat (first: First, second: Second): Result {
    return first.concat(second); // error
}

function spread (first: First, second: Second): Result {
    return [...first, ...second]; // ok
}

Expected behavior: concat function works similarly to spread function without an error.

Actual behavior: Type '(string | number)[]' is not assignable to type '[number, string, string, string]'. Target requires 4 element(s) but source may have fewer.(2322)

Playground Link:
playground

Related Issues: #36554

[RyanCavanaugh]

We might consider defining an overload:

interface Array<T> {
    concat2<S extends unknown[], U extends unknown[]>(this: S, other: U): [...S, ...U];
}

type First = [number, string];
type Second = [string, string];
type Result = [...First, ...Second];

function concat (first: First, second: Second): Result {
    return first.concat2(second);
}