c++11 - Why can't I specialize std::tuple_element? -

the following program attempts provide specialization std::tuple_element user-defined type foo. unfortunately, clang-3.5 rejects libc++, using other compilers or using other standard libraries clang-3.5 accept program. correct? if no, why not?

#include <utility>  struct foo {};  namespace std {     template<size_t, class> struct tuple_element;     template<size_t i>     struct tuple_element<i, foo> {}; }  int main() {     return 0; } 

compiler output:

$ clang-3.5 -std=c++11 -stdlib=libc++ -lc++ test.cpp test.cpp:11:8: error: explicit specialization of non-template struct 'tuple_element' struct tuple_element<i, foo> {};        ^            ~~~~~~~~ 1 error generated.  $ clang-3.5 -std=c++11 -lstdc++ test.cpp (no error)  $ g++-4.9 -std=c++11 test.cpp (no error) 

libc++'s entities in std::__1::, inline namespace inside std. own forward declaration template<size_t, class> struct tuple_element; declares different class template, , partial specialization blows due ambiguity (though error message misleading).

remove forward declaration , should work.