Generic type constraint does not apply to rest parameters

[sccolbert]

The following fails to compile even though T is constrained to an array type:

function attach<T extends any[]>(cb: (...args: T) => void): void { }

A rest parameter must be of an array type.
(parameter) args: T extends any[]

[danquirk]

This is technically by design but it's a reasonable suggestion. We also don't allow subtypes of array without generics.

[RyanCavanaugh]

Spec section 3.8.2.2

[RyanCavanaugh]

This doesn't actually make sense.

The function should be rewritten this way:

function attach<T>(cb: (...args: T[]) => void): void { }

Any production that requires that you write T instead of T[] would require a lie, because the type of the rest arg is always going to be the basic Array type, not some subtype thereof. It would be especially dangerous to flow that type through.

[JoshuaKGoldberg]

@RyanCavanaugh what if the function is meant to be used like the following?

attach((age: number, name: string) => {
    console.log({ age, name });
});

// written explicitly:
attach<[number, string]>(/*... */);

Using the ...args: T[] approach we would then have this typed as (number | string)[] instead of [number, string] right?

[RyanCavanaugh]

If that's the desired behavior you'd be better off writing some explicit overloads e.g. https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/underscore/underscore.d.ts#L1126

[ds300]

As a library author this makes me sad, since with overloading you lose the parameter name information. Plus it produces ugly inline documentation.

[YoshiWalsh]

I'd love to see this decision reconsidered. As I understand it, one of TypeScript's major design goals is to have a type system flexible enough to represent any common JavaScript design pattern. Rest parameters have been supported for years, and generics are a vital part of TypeScript, the fact that you can't use both together is unfortunate.