Add test for mp_empty with value lists. Refs #53.

test/Jamfile

@@ -28,6 +28,7 @@ compile mp11.cpp ;
 run mp_size.cpp ;
 run mp_size_2.cpp ;
 run mp_empty.cpp ;
+run mp_empty_2.cpp ;
 run mp_front.cpp ;
 run mp_pop_front.cpp ;
 run mp_second.cpp ;

test/mp_empty_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;
+    using boost::mp11::mp_empty;
+    using boost::mp11::mp_true;
+    using boost::mp11::mp_false;
+
+    BOOST_TEST_TRAIT_SAME(mp_empty<L1<>>, mp_true);
+    BOOST_TEST_TRAIT_SAME(mp_empty<L1<false>>, mp_false);
+    BOOST_TEST_TRAIT_SAME(mp_empty<L1<false, 0>>, mp_false);
+    BOOST_TEST_TRAIT_SAME(mp_empty<L1<false, 0, std::size_t(0)>>, mp_false);
+
+    BOOST_TEST_TRAIT_SAME(mp_empty<L2<>>, mp_true);
+    BOOST_TEST_TRAIT_SAME(mp_empty<L2<0>>, mp_false);
+    BOOST_TEST_TRAIT_SAME(mp_empty<L2<0, 1>>, mp_false);
+    BOOST_TEST_TRAIT_SAME(mp_empty<L2<0, 1, 2>>, mp_false);
+
+    return boost::report_errors();
+}
+
+#endif