Accessors of unions return dangling pointers for reinterpreted unions

[lucic71]

In refcount, when the union is reinterpreted, saving the pointer returned by a union accessor is resulting in a dangling pointer. For example:

/*
union U {
  int i;
  float f;
};
*/
pub struct U {
    __bytes: Value<Box<[u8]>>,
}

impl Number {
    pub fn i(&self) -> Ptr<i32> {
        (self.__bytes.as_pointer() as Ptr<u8>).reinterpret_cast()
    }
    pub fn f(&self) -> Ptr<f32> {
        (self.__bytes.as_pointer() as Ptr<u8>).reinterpret_cast()
    }
}

fn foo() {
  let p: Ptr<...> = ...;
  let reinterpreted_u: Ptr<U> = p.reinterpret_cast();
  let p_i: Ptr<i32> = reinterpreted_u.upgrade().deref().i();
  ...
  // p_i is dangling because reinterpreted_u.deref().upgrade() is creating a temporary union
}