+ // Core is assumed to be convertible only if the type is convertible
+ // and the size is the same. This is a compromise for the complexity
+ // of having to make Core truly have a conversion constructor which
+ // would cause various other problems.
+ // If we made Core move constructible then we would need to update the
+ // Promise and Future with the location of the new Core. This is complex
+ // and may be inefficient.
+ // Core should only be modified so that for size(T) == size(U),
+ // sizeof(Core<T>) == size(Core<U>).
+ // This assumption is used as a proxy to make sure that
+ // the members of Core<T> and Core<U> line up so that we can use a
+ // reinterpret cast.
+ template <
+ class U,
+ typename = typename std::enable_if<std::is_convertible<U, T>::value &&
+ sizeof(U) == sizeof(T)>::type>
+ static Core<T>* convert(Core<U>* from) {
+ return reinterpret_cast<Core<T>*>(from);
+ }
+