Optional

I'm working on an Optional<T> class because I'm using c++11 and the compiler does not provide an implementation. I have problem with the rvalue reference constructor. The following constructors seems to work fine

Optional();

Optional(const ThisType& other);
Optional(ThisType&& other);

template <typename U>
Optional(const Optional<U>& other);
template <typename U>
Optional(Optional<U>&& other);

template <typename... Args>
explicit Optional(Unused, Args&&... args);
template <typename U, typename... Args>
explicit Optional(Unused, InitializerList<U> initList, Args&&... args);

template <typename U>
Optional(const U& value);

Where ValueType is a typedef T and ThisType is a typedef Optional<T>. However when I'm trying to implement an rvalue reference of U constructor:

template <typename U>
Optional(U&& value);

then the following test case is not working:

// Optional<T>(const Optional<T>&)
Opt o(bmstl::Unused{});
Opt o2 = o;

The compiler tries to resolve this using the Optional<T>(U&&) and it of course cannot convert Optional<T> to T, where Optional<T> is Opt and T is Foo

error C2664: 'Foo::Foo(Foo &&)' : cannot convert argument 1 from 'Opt' to 'const Foo &'

If I change the test case from Opt o(bmstl::Unused{}); to const Opt o(bmstl::Unused{}); it works fine, but that's not a solution.

I've tried to use enable_if on the rvalue constructor with no luck. I thought that I should enable this when the type of U is not an Optional<U> but probably I'm wrong.

template <typename U, typename = typename EnableIf<!IsSame<DecayT<U>, Optional<U>>::Value>::Type>
Optional(U&& value);