Use __type_pack_element in mp_at(_c)

include/boost/mp11/algorithm.hpp

@@ -264,20 +264,29 @@ template<class N> using mp_iota = detail::mp_from_sequence<make_integer_sequence
 namespace detail
 {
 
-template<class L, class I> struct mp_at_impl
+template<class L, std::size_t I> struct mp_at_c_impl;
+
+#if defined(BOOST_MP11_HAS_TYPE_PACK_ELEMENT)
+
+template<template<class...> class L, class... T, std::size_t I> struct mp_at_c_impl<L<T...>, I>
 {
-    static_assert( I::value >= 0, "mp_at<L, I>: I must not be negative" );
+    using type = __type_pack_element<I, T...>;
-
-    using _map = mp_transform<mp_list, mp_iota<mp_size<L>>, L>;
-
-    using type = mp_second<mp_map_find<_map, mp_size_t<I::value>>>;
 };
 
+#else
+
+template<class L, std::size_t I> struct mp_at_c_impl
+{
+    using _map = mp_transform<mp_list, mp_iota<mp_size<L>>, L>;
+    using type = mp_second<mp_map_find<_map, mp_size_t<I>>>;
+};
+
+#endif
+
 } // namespace detail
 
-template<class L, std::size_t I> using mp_at_c = typename detail::mp_at_impl<L, mp_size_t<I>>::type;
+template<class L, std::size_t I> using mp_at_c = typename detail::mp_at_c_impl<L, I>::type;
-
+template<class L, class I> using mp_at = typename detail::mp_at_c_impl<L, std::size_t{ I::value }>::type;
-template<class L, class I> using mp_at = typename detail::mp_at_impl<L, I>::type;
 
 // mp_take(_c)<L, N>
 namespace detail

include/boost/mp11/detail/config.hpp

@@ -29,4 +29,12 @@
 # endif
 #endif
 
+#if defined(__has_builtin)
+# if __has_builtin(__type_pack_element)
+
+#  define BOOST_MP11_HAS_TYPE_PACK_ELEMENT
+
+# endif
+#endif
+
 #endif // #ifndef BOOST_MP11_DETAIL_CONFIG_HPP_INCLUDED