Default impls cannot take into account associated types

[Ericson2314]

Suppose I have some trait with a default method:

trait Foo {
  fn bar(&self, isize) { .. }
}

and I want to add an associated type:

trait Foo {
  type Baz = isize;
  fn bar(&self, Self::Baz);
}

This is a breaking change unless I move the old default method to a new default impl. But there is no way to condition the default impl on choice the choice of associated type. For example:

default impl<A: Foo<Baz = isize>> Foo for A {
  fn bar(&self, isize) { .. }
}

is prohibited, and both

default impl<A> Foo for A {
  type Baz = isize;
  fn bar(&self, Self::Baz);
}

and

default impl<A> Foo for A {
  default type Baz = isize;
  fn bar(&self, Self::Baz);
}

do the wrong thing.