https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79960
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It's possible this isn't a bug, and the partial specializations need to be
constrained to avoid the ambiguity (although both Clang trunk and EDG accept
the unconstrained cases, but Clang 3.x fails similarly to G++).
This works with any G++ version:
using size_t = decltype(sizeof(0));
struct true_type { static constexpr bool value = true; };
struct false_type { static constexpr bool value = false; };
template<bool> struct enable_if { };
template<> struct enable_if<true> { using type = void; };
template<typename T> using disable_if = typename enable_if<!T::value>::type;
template<typename T> struct is_const : false_type { };
template<typename T> struct is_const<const T> : true_type { };
template<typename T> struct is_volatile : false_type { };
template<typename T> struct is_volatile<volatile T> : true_type { };
template<typename T> struct tuple_size;
template<typename T, typename = void, size_t U = tuple_size<T>::value>
using __has_tuple_size = T;
template<typename T>
struct tuple_size<const __has_tuple_size<T, disable_if<is_volatile<T>>>> {
static constexpr size_t value = tuple_size<T>::value;
};
template<typename T>
struct tuple_size<volatile __has_tuple_size<T, disable_if<is_const<T>>>> {
static constexpr size_t value = tuple_size<T>::value;
};
template<typename T>
struct tuple_size<const volatile __has_tuple_size<T>> {
static constexpr size_t value = tuple_size<T>::value;
};
template<typename... T> struct tuple { };
template<typename... T> struct tuple_size<tuple<T...>> {
static constexpr size_t value = sizeof...(T);
};
static_assert( tuple_size<const tuple<>>::value == 0, "" ); // OK
static_assert( tuple_size<volatile tuple<>>::value == 0, "" ); // OK
static_assert( tuple_size<const volatile tuple<>>::value == 0, "" ); // OK
