Proposal: Support spread operator for tuples in function calls

[jp-hoehmann]

With current versions of TypeScript, this is an error:

((_) => {})(...[null]); // ERROR: Expected 1 arguments, but got a minimum of 0.

In order to make this code work, the developer needs to tell the type checker to shut up, which could look like this:

(((_) => {}) as any)(...[null]); // Working fine.

This is unfortunate, as passing arguments for functions around in tuples is a common pattern in functional programming and there is currently no way to do it safely in TypeScript.

This was mentioned in #5296, which lead to the spread operator being supported for arrays. Supporting tuples in addition to arrays is not a straightforward change, since tuples can currently have additional values, which are not type checked (there is proposal #6229, which aims to fix this):

function f(s: string, a: number, b: number, c: number,  ...rest: number[]) {
    // ...
}
// legal
let quad: [string, number, number, number, number] = ["foo", 1,2,3,4];
f(...quad); // and triple

quad = ["foo", 1, 2, 3, 4, "boo!"]; // it is legal currently
f(...quad); // now it blows up in run time because rest has a string

Here is a longer and more detailed example taken from #5296 showing the desired behavior in detail:

function f(s: string, a: number, b: number, c: number,  ...rest: number[]) {
    // ...
}
function g(s: string, a: number, b: number) {
}
// legal
let quad: [string, number, number, number, number] = ["foo", 1,2,3,4];
let triple: [string, number, number, number] = ["foo", 1,2,3];
let double: [string, number, number] = ["foo", 1,2];
f(...quad); // and triple
f(...double, 3);
f("foo", ...[1,2,3]);
f(...quad, 1, ...[1,2,3]); // and triple
g(...double);
// illegal
f(...double); // too short -- double doesn't match parameter 'c'
g(...triple); // too many parameters
g(...quad); // too many parameters

[ikatyang]

Seems related #5453 #17765 #17884 #17898.

[KiaraGrouwstra]

The strict tuples should help, but wouldn't be sufficient; the request seems legitimate. afaik the code did have a check so as to match spreads in rest param positions, but yeah this may well be missing.

[mhegazy]

duplicate of #4130?

[jp-hoehmann]

@mhegazy whoops... didn't notice that one was still open. I'll close this.