Dart compilers reify Null and bottom inconsistently

[vsmenon]

Here are a couple examples:

class A {
  var x = () => throw 'Hi';
  var y = () => null;
}

Null f() => null;
main() {
  A a = new A();
  print(a.x.runtimeType);
  print(a.y.runtimeType);
  a.y = a.x;
  // a.x = f;  Static error in CFE, ok in analyzer
  a.x = (f as dynamic);
  print(a.x.runtimeType);
  print(a.y.runtimeType);
}

On VM and Dart2JS, this prints:

() => Null
() => Null
() => Null
() => Null

On DDC and DDK, this prints:

() => bottom
() => Null
() => Null
() => bottom

For this example:

void Function(E) eventHandler<E, F extends E>(void Function(F) handler) {
  return (E e) {
    print(E);
    print(F);
  };
}

void main() {
  void Function(int) f = eventHandler((String s) {});
  f(42);
}

On VM, Dart2JS, and DDC:

int
Null

On DDK:

int
bottom

The simplest answer is to always reify as Null regardless of what the CFE or Analyzer passes in. If so, we can change DDC / DDK accordingly.

[vsmenon]

@sigmundch @alexmarkov - do Dart2JS and/or the VM ever reify bottom? Or do they just map it to Null?

fyi - @leafpetersen

[leonsenft]

The simplest answer is to always reify as Null regardless of what the CFE or Analyzer passes in. If so, we can change DDC / DDK accordingly.

I think this is necessary as bottom isn't a user visible type (at least not yet), so there's no way to test for it like you can for Null.

[alexmarkov]

VM maps kernel Bottom type to Null when reading dill files. There is no runtime representation for bottom type in VM.

[sigmundch]

dart2js as well, we map bottom to null and there is no reified notion of bottom.

I agree that DDC/DDK should do the same.

[vsmenon]

Thanks! Let's go forward with the DDC/K change to be consistent.