Please take a look at the code below, sorry this is a little longer, but I did my best to reproduce the problem with a minimal example (there is also a live copy ). There I have a metafunction that returns the size of a string literal and a constexpr function that wraps it. Then, when I call these functions in the gcc template parameter (5.4, 6.2), he is satisfied, but clang (3.8, 3.9) barfs with the "non-standard template argument is not a constant expression" in the test for strsize(s) . If I replace with str_size<S> , both compilers will be happy. So the questions are:
is there a problem with clang or my code?
How can I compile both clang and gcc using the constexpr function?
template<size_t N> using string_literal_t = char[N]; template<class T> struct StrSize; ///< metafunction to get the size of string literal alikes /// specialize StrSize for string literals template<size_t N> struct StrSize <string_literal_t<N>>{ static constexpr size_t value = N-1; }; /// template variable, just for convenience template <class T> constexpr size_t str_size = StrSize<T>::value; /// now do the same but with constexpr function template<class T> constexpr auto strsize(const T&) noexcept-> decltype(str_size<T>) { return str_size<T>; } template<class S, size_t... Is> constexpr auto test_helper(const S& s, index_sequence<Is...>) noexcept-> array<char, str_size<S>> { return {s[Is]...}; } template<class S> constexpr auto test(const S& s) noexcept-> decltype(auto) { // return test_helper(s, make_index_sequence<str_size<S>>{}); // this work in both clang and gcc return test_helper(s, make_index_sequence<strsize(s)>{}); // this works only in gcc } auto main(int argc, char *argv[])-> int { static_assert(strsize("qwe") == 3, ""); static_assert(noexcept(test("qwe")) == true, ""); return 0; }
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