Flow: `Class` should be invariant

[vkurchatkin]

Example:

class A {}

class B extends A {
  constructor(a: number) {
    super();
    a.toFixed();
  }
}

const a: Class<B> = B;
const b: Class<A> = a;

const x = new b(); // no error

[vkurchatkin]

Related: https://github.com/facebook/flow/issues/1047

[jeffmo]

This looks like a bug in that the constructor of B shouldn't be permitted (it is incompatible with A's constructor).

Consider an example with a normal method and you can see that Flow correctly errors:

class A {
  foo() {}
}

class B extends A {
  foo(a: number) {}
}

https://flowtype.org/try/#0MYGwhgzhAECC0G8BQ1oDMD2GAUBKRAvkkUqJDAELQCmAHgC7UB2AJjPMqpjmAFzRMArgFsARtQBO+BEQJA

Overall, Class<> should be covariant because static-classes in JS themselves are an "instance" of the static-class they extend.

[vkurchatkin]

It is very common in OOP languages to change constructor in subclasses arbitrarily (e.g. provide default args), so it will be very inconvenient.

Another option is not to consider class B subtype of class A if constructor B is not a subtype of constructor of A.

[jeffmo]

Another option is not to consider class B subtype of class A if constructor B is not a subtype of constructor of A.

I like this approach