Strange behaviour with 'void' as function argument

[lifaon74]

TypeScript Version: 3.4.5, 3.5.0-rc and next

Search Terms:
void, function, argument, template

Code

class A<T> {
  constructor(a: T) {
  }
}

class B {
  constructor(a: void) {
  }
}


const a = new A<void>(); // error ! => variable 'a' should be optional !
const b = new B();

Expected behavior:
It's pretty hard to understand why with a template function having a 'void' type (in class A), the behaviour is not the same as the class B.
Allowing a developper to put 'void' as template and providing 0 argument is sometimes required.

The following 'solution' is not acceptable, because the variable a would always be optional where I want to have it optional only if T is void.

class A<T> {
  constructor(a?: T) {
  }
}

Actual behavior:
class A diverge from B even if the typing is conceptualy the same.

Playground Link:
https://www.typescriptlang.org/play/#src=class%20A%3CT%3E%20%7B%0D%0A%20%20constructor(a%3A%20T)%20%7B%0D%0A%20%20%7D%0D%0A%7D%0D%0A%0D%0Aclass%20B%20%7B%0D%0A%20%20constructor(a%3A%20void)%20%7B%0D%0A%20%20%7D%0D%0A%7D%0D%0A%0D%0A%0D%0Aconst%20a%20%3D%20new%20A%3Cvoid%3E()%3B%20%2F%2F%20error%20!%0D%0Aconst%20b%20%3D%20new%20B()%3B

More examples:

type CType<T> = T extends any[] ? T : void;

class C<T> {
  constructor(a: CType<T>) {
  }
}

const c = new C<number>(); // error !

function foo<T>(a: T): void {
}

foo<void>(); // error !

[jack-williams]

Duplicate of #29131.

The short answer is that arity checking that accounts for void is done before generic instantiation.

[nmain]

Workaround:

type Args<T> = T extends void ? [T?] : [T];

function foo<T>(...args: Args<T>): void;
function foo<T>(arg: T) { // separate declaration from implementation, so the implementation doesn't need to use rest params
}

foo<void>(); // works!
foo<number>(); // fails as expected
foo<number>(3); // works!

https://www.typescriptlang.org/play/#src=type%20Args%3CT%3E%20%3D%20T%20extends%20void%20%3F%20%5BT%3F%5D%20%3A%20%5BT%5D%3B%0D%0A%0D%0Afunction%20foo%3CT%3E(...args%3A%20Args%3CT%3E)%3A%20void%3B%0D%0Afunction%20foo%3CT%3E(arg%3A%20T)%20%7B%20%2F%2F%20separate%20declaration%20from%20implementation%2C%20so%20the%20implementation%20doesn't%20need%20to%20use%20rest%20params%0D%0A%7D%0D%0A%0D%0Afoo%3Cvoid%3E()%3B%20%2F%2F%20works!%0D%0Afoo%3Cnumber%3E()%3B%20%2F%2F%20fails%20as%20expected%0D%0Afoo%3Cnumber%3E(3)%3B%20%2F%2F%20works!%0D%0A%0D%0A

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.