Add support for value lists to mp_front. Refs #53.

include/boost/mp11/detail/mp_front.hpp

@@ -1,13 +1,16 @@
 #ifndef BOOST_MP11_DETAIL_MP_FRONT_HPP_INCLUDED
 #define BOOST_MP11_DETAIL_MP_FRONT_HPP_INCLUDED
 
-//  Copyright 2015-2021 Peter Dimov.
+//  Copyright 2015-2023 Peter Dimov.
 //
 //  Distributed under the Boost Software License, Version 1.0.
 //
 //  See accompanying file LICENSE_1_0.txt or copy at
 //  http://www.boost.org/LICENSE_1_0.txt
 
+#include <boost/mp11/detail/mp_value.hpp>
+#include <boost/mp11/detail/config.hpp>
+
 namespace boost
 {
 namespace mp11
@@ -28,6 +31,15 @@ template<template<class...> class L, class T1, class... T> struct mp_front_impl<
     using type = T1;
 };
 
+#if defined(BOOST_MP11_HAS_TEMPLATE_AUTO)
+
+template<template<auto...> class L, auto A1, auto... A> struct mp_front_impl<L<A1, A...>>
+{
+    using type = mp_value<A1>;
+};
+
+#endif
+
 } // namespace detail
 
 template<class L> using mp_front = typename detail::mp_front_impl<L>::type;

test/Jamfile

@@ -34,6 +34,7 @@ run mp_assign_2.cpp ;
 run mp_clear.cpp ;
 run mp_clear_2.cpp ;
 run mp_front.cpp ;
+run mp_front_2.cpp ;
 run mp_pop_front.cpp ;
 run mp_second.cpp ;
 run mp_third.cpp ;

test/mp_front_2.cpp

@@ -0,0 +1,40 @@
+// Copyright 2023 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/mp11/list.hpp>
+
+#if !defined(BOOST_MP11_HAS_TEMPLATE_AUTO)
+
+#pragma message("Test skipped because BOOST_MP11_HAS_TEMPLATE_AUTO is not defined")
+int main() {}
+
+#else
+
+#include <boost/mp11/integral.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+
+template<auto... A> struct L1 {};
+template<int... I> struct L2 {};
+
+int main()
+{
+    using boost::mp11::mp_list_v;
+    using boost::mp11::mp_front;
+    using boost::mp11::mp_first;
+    using boost::mp11::mp_false;
+    using boost::mp11::mp_int;
+
+    BOOST_TEST_TRAIT_SAME(mp_front<mp_list_v<false>>, mp_false);
+    BOOST_TEST_TRAIT_SAME(mp_first<mp_list_v<false>>, mp_false);
+
+    BOOST_TEST_TRAIT_SAME(mp_front<L1<-1, true>>, mp_int<-1>);
+    BOOST_TEST_TRAIT_SAME(mp_first<L1<-1, true>>, mp_int<-1>);
+
+    BOOST_TEST_TRAIT_SAME(mp_front<L2<0, 1>>, mp_int<0>);
+    BOOST_TEST_TRAIT_SAME(mp_first<L2<0, 1>>, mp_int<0>);
+
+    return boost::report_errors();
+}
+
+#endif