I've noticed that clang++ accepts, but g++ rejects:
namespace std { template<typename T> struct tuple_size; }
struct A { int x, y, z; };
template<> struct std::tuple_size<A> { static const int value = 3; };
constexpr int a = std::tuple_size<A>::value;
test.C:3:24: error: specialization of ‘template<class T> struct std::tuple_size’ in different namespace [-fpermissive]
 template<> struct std::tuple_size<A> { static const int value = 3; };
                        ^~~~~~~~~~~~~
test.C:1:45: error:   from definition of ‘template<class T> struct std::tuple_size’ [-fpermissive]
 namespace std { template<typename T> struct tuple_size; }
                                             ^~~~~~~~~~
Is that a g++ or clang++ bug?