Invalid use of incomplete type struct std::hash with unordered_map with std::pair of enum class as key

I want to use a unordered_map<std::pair<enum_class,other_enum_class>,std::uint8_t> for managing some pixelmap formats.

Here the minimal code :

#include <unordered_map>
#include <utility>
#include <cstdint> 
#include <iostream>
#include <functional>

enum class PNM : std::uint8_t { PBM, PGM, PPM };
enum class Format : bool      { BIN, ASCII };

struct pair_hash {
public:
    template <typename T, typename U>
    std::size_t operator()(const std::pair<T, U> &x) const { 
        return std::hash<T>()(x.first) ^ std::hash<U>()(x.second); 
    }
};

int main(){

    std::unordered_map<std::pair<PNM, Format>, std::uint8_t, pair_hash> k_magic_number ({
        { { PNM::PBM, Format::BIN   }, 1 }, { { PNM::PGM, Format::BIN   }, 2 }, { { PNM::PPM, Format::BIN   }, 3 },
        { { PNM::PBM, Format::ASCII }, 4 }, { { PNM::PGM, Format::ASCII }, 5 }, { { PNM::PPM, Format::ASCII }, 6 }
    });

    std::cout << k_magic_number[std::make_pair<PNM, Format>(PNM::PBM, Format::BIN)];
}

With GCC I have an error when I try to instantiate the class :

main.cpp:14:24: error: invalid use of incomplete type 'struct std::hash'
return std::hash()(x.first) ^ std::hash()(x.second);
In file included from
/usr/local/include/c++/5.2.0/bits/basic_string.h:5469:0,
from /usr/local/include/c++/5.2.0/string:52,
[...]

With Clang I also have an error :

error: implicit instantiation of undefined template 'std::hash' return std::hash()(x.first) ^ std::hash()(x.second); /usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.2.0/../../../../include/c++/5.2.0/bits/hashtable_policy.h:1257:16: note: in instantiation of function template specialization 'pair_hash::operator()' requested here [...]

With VS2013 I have no error and the code compile and executes.

What is missing in my code ?