External callable type with a ref parameter can't be assigned to

[masonwheeler]

Assembly 1:

callable FooDelegate(ref int)

Assembly 2: class Foo: private del as FooDelegate

   private def DoFoo(ref int):
      pass

   def constructor():
      del = self.DoFoo

Expected: This should work Observed: Cannot convert 'callable(ref int) as void' to 'FooDelegate'. (BCE0022)