Invalid type error for protected constructor

[jonaaix]

If I create an abstract class, with an explicitly protected constructor, I'm getting a type error, though a protected constructor is the default for abstract classes (afaik)

export abstract class UrlFilterHandler {
   protected constructor() {
      // this.init();
   }
}

[andres-flores-espol]

The type definition of a class in TypeScript declares that it must be able to expose abstract new(...args: any[]) => any in its interface. However, a class whose constructor is protected, although a class, is not recognized as such given this definition. It is also possible to define it as a 'protected class', meaning "an object that exposes prototype in its interface but does not expose abstract new(...args: any[]) => any."

We would then have something like

type PublicClass<
	A extends any[] = any[],
	I = {},
	S = {},
> =
	(abstract new (...args: A) => I) & S;

type ProtectedClass<
	A extends any[] = any[],
	I = {},
	S = {},
> =
	I extends PublicClass
	? never
	: S extends PublicClass
	? never
	: ({ prototype: I; } & S);

type Class<
	A extends any[] = any[],
	I = {},
	S = {},
> =
	PublicClass<A, I, S> | ProtectedClass<A, I, S>;

And if we test with some structures like

class Foo {
	public constructor() { }
}

class Bar {
	protected constructor() { }
}

const Obj = {
	prototype: {}
};

function Fun() {

}

type CheckFoo = typeof Foo extends Class ? true : false; // typeof Foo extends (abstract new () => Foo) & typeof Foo = true
type CheckBar = typeof Bar extends Class ? true : false; // typeof Bar extends (abstract new () => Bar) & typeof Bar = true
type CheckObj = typeof Obj extends Class ? true : false; // typeof Obj extends (abstract new (...args: any[]) => {}) & { prototype: {}; } = true
type CheckFun = typeof Fun extends Class ? true : false; // typeof Fun extends (abstract new (...args: any[]) => any) & (() => void) = true, since functions have prototype: any

Therefore, even though functions can be interpreted as classes, it is still possible to validate them when checking the Mixin type.