Containers: provide interface for converting Optional to something else.

Will be used by Magnum to provide backwards compatibility with
(previously bundled) std::optional. I hated myself for bundling
std::optional, now I will hate myself for doing this.

src/Corrade/Containers/Optional.h

@@ -33,6 +33,10 @@
 
 namespace Corrade { namespace Containers {
 
+#ifdef CORRADE_BUILD_DEPRECATED
+namespace Implementation { template<class, class> class OptionalConverter; }
+#endif
+
 /**
 @brief Null optional initialization tag type
 
@@ -254,6 +258,17 @@ template<class T> class Optional {
          */
         template<class ...Args> T& emplace(Args&&... args);
 
+        #if defined(CORRADE_BUILD_DEPRECATED) && !defined(CORRADE_GCC47_COMPATIBILITY)
+        /* Used by Magnum to provide backwards compatibility with (previously
+           bundled) std::optional. I hate myself for this. */
+        template<class U, class V = decltype(Implementation::OptionalConverter<T, U>::to(std::declval<Optional<T>>()))> operator U() const & {
+            return Implementation::OptionalConverter<T, U>::to(*this);
+        }
+        template<class U, class V = decltype(Implementation::OptionalConverter<T, U>::to(std::declval<Optional<T>>()))> operator U() && {
+            return Implementation::OptionalConverter<T, U>::to(std::move(*this));
+        }
+        #endif
+
     private:
         union Storage {
             constexpr Storage() noexcept: _{} {}