Implement type_name on GCC and fix on older Clangs

Author: ldionne

include/boost/hana/experimental/type_name.hpp

@@ -26,18 +26,23 @@ namespace boost { namespace hana { namespace experimental {
 
         // Note: We substract the null terminator from the string sizes below.
         template <typename T>
-        constexpr cstring type_name_impl2() {
-
-        #if defined(__clang__)
+        constexpr auto type_name_impl2() {
+        #if defined(BOOST_HANA_CONFIG_CLANG)
+            constexpr char const* pretty_function = __PRETTY_FUNCTION__;
+            constexpr std::size_t total_size = sizeof(__PRETTY_FUNCTION__) - 1;
+            constexpr std::size_t prefix_size = sizeof("auto boost::hana::experimental::detail::type_name_impl2() [T = ") - 1;
+            constexpr std::size_t suffix_size = sizeof("]") - 1;
+        #elif defined(BOOST_HANA_CONFIG_GCC)
             constexpr char const* pretty_function = __PRETTY_FUNCTION__;
             constexpr std::size_t total_size = sizeof(__PRETTY_FUNCTION__) - 1;
-            constexpr std::size_t prefix_size = sizeof("cstring boost::hana::experimental::detail::type_name_impl2() [T = ") - 1;
+            constexpr std::size_t prefix_size = sizeof("constexpr auto boost::hana::experimental::detail::type_name_impl2() [with T = ") - 1;
             constexpr std::size_t suffix_size = sizeof("]") - 1;
         #else
             #error "No support for this compiler."
         #endif
 
-            return {pretty_function + prefix_size, total_size - prefix_size - suffix_size};
+            cstring s{pretty_function + prefix_size, total_size - prefix_size - suffix_size};
+            return s;
         }
 
         template <typename T, std::size_t ...i>