Bad error message when trying to define apply in an anonymous class extending js.Function

[gzm0]

While trying to break our detection of JS functions, I found this:

@js.native
@JSGlobal("bad")
abstract class BadFunction extends js.Function1[Int, String]

object Test {
  new BadFunction {
    def apply(x: Int): String = "f"
  }
}

This fails to compile with the following error:

Concrete members of JS native types may only call js.native.
      def apply(x: Int): String = "f"
                                  ^

Note that this does not happen for other native traits:

@js.native
trait MyTrait[T] extends js.Object {
  def apply(): T
}

@js.native
@JSGlobal("bad")
abstract class BadMyTrait extends MyTrait[Int]

object Test {
  new BadMyTrait {
    def apply(): Int = 1
  }
}

This fails as expected with:

A non-native JS class cannot declare a method named `apply` without `@JSName`
      def apply(): Int = 1
          ^

[sjrd]

We should not even allow extending any subclass of any js.FunctionN.

[gzm0]

We should not even allow extending any subclass of any js.FunctionN.

As long as all that stuff stays native, it's probably OK? Or would the expectation for a facade be to define it's own applys? (FWIW, js.Object's compation does that without extending the relevant js.FunctionNs or even js.Function).

[sjrd]

I mean, it's not really possible for a class, native or not, to truly extend a js.FunctionN. One cannot create any kind of class (with class or function) whose instances would be callable. Native objects can meaningfully provide an apply because they're functions themselves (they are not of a class that extend Function); native traits can define an apply, because in practice that means it's a function on which we've patched the other members of the interface. But a class can never provide an apply.

[sjrd]

Hum ... it appears there is a way to define a class that could provide a call syntax:

>node
Welcome to Node.js v14.15.0.
Type ".help" for more information.
> class Foo extends Function { constructor(...args) { super(...args); } }
undefined
> var f = new Foo("x", "return x + 1;");
undefined
> f(5)
6
> typeof f
'function'

[gzm0]

Maybe, for now, we should simply disallow non-native subclasses of js.FunctionN?

[sjrd]

Yes, that sounds reasonable.