Lambda expressions cannot appear in unevaluated operands nor in evaluated portions of constant expressions. However, the following example appears to circumvent those restrictions:
template <bool> struct BoolSink { typedef void type; }; template <typename T, typename U> struct AddRvalueReferenceImpl { typedef T type; }; template <typename T> struct AddRvalueReferenceImpl<T, typename BoolSink<false && [] { extern T &&tref; }>::type> { typedef T &&type; }; template <typename T> struct AddRvalueReference : AddRvalueReferenceImpl<T, void> { }; namespace ImplHelpers { template <typename T> typename AddRvalueReference<T>::type create(void) { } } template <typename T, typename U, typename ...Args> struct IsConstructibleImpl { enum { value = 0 }; }; template <typename T, typename ...Args> struct IsConstructibleImpl<T, typename BoolSink<false && [] { T t( ::ImplHelpers::create<Args>() ...); }>::type, Args ...> { enum { value = 1 }; }; template <typename T, typename ...Args> struct IsConstructible : IsConstructibleImpl<T, void, Args ...> { }; struct DestroyMe { ~DestroyMe() = delete; }; static_assert(+IsConstructible<int>::value, "error"); static_assert(!IsConstructible<void>::value, "error"); static_assert(+IsConstructible<int [1]>::value, "error"); static_assert(!IsConstructible<DestroyMe>::value, "error"); static_assert(!IsConstructible<int *, char *>::value, "error"); static_assert(+IsConstructible<int &&, int>::value, "error"); static_assert(!IsConstructible<int &&, int &>::value, "error"); static_assert(+IsConstructible<int &&, int &&>::value, "error");
Is this intended?