constexpr to concatenate two or more char strings

I'm trying to make a constexpr function that will concatenate an arbitrary number of char arrays by working from the following answer by Xeo, which concatenates two char arrays.

http://ift.tt/1zEQW2K

#include <array>

template<unsigned... Is> struct seq{};
template<unsigned N, unsigned... Is>
struct gen_seq : gen_seq<N-1, N-1, Is...>{};
template<unsigned... Is>
struct gen_seq<0, Is...> : seq<Is...>{};

template<unsigned N1, unsigned... I1, unsigned N2, unsigned... I2>
constexpr std::array<char const, N1+N2-1> concat(char const (&a1)[N1], char const (&a2)[N2], seq<I1...>, seq<I2...>){
  return {{ a1[I1]..., a2[I2]... }};
}

template<unsigned N1, unsigned N2>
constexpr std::array<char const, N1+N2-1> concat(char const (&a1)[N1], char const (&a2)[N2]){
  return concat(a1, a2, gen_seq<N1-1>{}, gen_seq<N2>{});
}

My attempt thus far:

#include <iostream>
#include <array>

template<unsigned... Is> struct seq{};
template<unsigned N, unsigned... Is>
struct gen_seq : gen_seq<N-1, N-1, Is...>{};
template<unsigned... Is>
struct gen_seq<0, Is...> : seq<Is...>{};

template<unsigned N1, unsigned... I1, unsigned N2, unsigned... I2>
constexpr const std::array<char, N1+N2-1>
concat_impl(
    const char (&a1)[N1], const char (&a2)[N2], seq<I1...>, seq<I2...>)
{
    return {{ a1[I1]..., a2[I2]... }};
}

template<unsigned N1, unsigned N2>
constexpr const std::array<char, N1+N2-1>
concat(const char (&a1)[N1], const char (&a2)[N2])
{
    return concat_impl(a1, a2, gen_seq<N1-1>{}, gen_seq<N2>{});
}

template<unsigned N1, unsigned N2, class... Us>
constexpr auto
concat(const char(&a1)[N1], const char(&a2)[N2], const Us&... xs)
-> std::array<char, N1 + decltype(concat(a2, xs...))::size() - 1>
{
    return concat(a1, concat(a2, xs...));
}

int main()
{
    auto const s = concat("hi ", "there!");
    std::cout << s.data() << std::endl;
    // compile error:
    auto const t = concat("hi ", "there ", "how ", "are ", "you?");
    std::cout << t.data() << std::endl;
}

Both gcc 4.9 and clang 3.5 give errors indicating that no function matching the concat inside the decltype expression can be found.

clang:

error: no matching function for call to 'concat'
    auto const t = concat("hi ", "there ", "how ", "are ", "you?");
                   ^~~~~~
ctconcat.cpp:105:16: note: candidate template ignored: substitution failure [with N1 = 4, N2 = 7, Us = <char [5], char [5], char [5]>]: no matching function for call to 'concat'
constexpr auto concat(const char(&a1)[N1], const char(&a2)[N2], const Us&... xs) -> std::array<char, N1 + decltype(concat(a2, xs...))::size() - 1>
               ^                                                                                                   ~~~~~~
ctconcat.cpp:62:43: note: candidate function template not viable: requires 2 arguments, but 5 were provided
constexpr const std::array<char, N1+N2-1> concat(const char (&a1)[N1], const char (&a2)[N2])
                                          ^
1 error generated.

The errors from gcc and clang both indicate that the second concat function template is not a candidate for the concat in the decltype expression. Only the first template is considered. Why is that and how do I fix this?