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)

Maury

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.

Deena

Replacing struct with class fixes a warning as well. Also note that the forward declaration makes the program ill-formed, as you are free to specialize types in std, but any declaration is verboten (even a forward declaration!). The build break is compliant. – Cupreous

@Yakk s/ill-formed/UB/, to be pedantic. – Deena

So, are you saying the code from the original post is ill-formed? In other words, it's not legal to provide declarations of standard library components? – Maury

17.6.4.2.1 Namespace std [namespace.std]/1 -- your code adds a declaration to namespace std. Behavior is thus undefined. – Cupreous