Add Support for Opaque Polymorphic Types

[ajbeach]

I have been trying to make some smart pointers and I hit a problem with the polymorphic data structures. All are polymorphic on a type T which is the type they point at (their dereference returns a T&).

One of the pointers points directly at a T object and it works as a pointer and T can be opaque. The other one wraps the T type in a new structure. This means it must have a sized assertion and in turn the pointer type must also include it, which means it cannot be used with an opaque.

To explain it here are the current definitions:

forall(dtype T | sized(T))
struct counter_data {
    unsigned int counter;
    T object;
};

forall(dtype T | sized(T) /* ! */)
struct counter_ptr {
    counter_data(T) * data;
};

The ! assertion is the one that means you can't use an opaque type. This conflicts with the traditional C use of a pointer to an opaque type if you wanted to pass around a memory managed pointer.

The problem is that all uses of a polymorphic type must carry its assertions, even if they aren't used here. There should be a way to drop sized from the assertion list when you just have a pointer to the type. The system doesn't have to know the size or layout as long as you don't dereference the pointer except for the assertion list.