Custom utility types (generic types) for classes `IsClass` of TypeScript

Question

I am trying to create a generic type to make sure the first parameter to be a class. However, the factory function parameter cannot be replaced by a generic type.

The following upper parts were my attempts. The last part were a working example that directly write the extends ... which worked.

Why does it works inside a function, but not works as a generic type IsClass?

class A {
  constructor() {

  }
}
class B {}
class C extends B {}

// ERRORS:

type IsClass<T extends new (...args: any) => InstanceType<T>> = T
type IsClass2<T> = T extends new (...args: any) => InstanceType<T>? T: never

type X = IsClass<A>
type Y = IsClass2<A>

function someFactoryError<T>(clx: IsClass<T>) {
  return new clx()
}

someFactoryError(A)

function someFactoryError2<T>(clx: IsClass2<T>) {
  return new clx()
}

someFactoryError2(A)

// WORKS:

function someFactoryWorks<T extends new (...args: any) => InstanceType<T>>(clx: T) {
  return new clx()
}

const a0 = someFactoryWorks(A)
const b0 = someFactoryWorks(B)

TypeScript Playground

Related:

• stackoverflow
• TypeScript Handbook: Abstract Construct Signatures