Why can't I specialize std::tuple_element?

Question

The following program attempts to provide a specialization for std::tuple_element for the user-defined type foo. Unfortunately, clang-3.5 rejects it with libc++, but using other compilers or using other standard libraries with clang-3.5 accept the program. Is it 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)

Answer 1

libc++'s entities are actually in std::__1::, an inline namespace inside std. So your own forward declaration template<size_t, class> struct tuple_element; actually declares a different class template, and then the partial specialization blows up due to ambiguity (though the error message is misleading).

Remove the forward declaration and it should work.