Add a test for mp_back with value lists. Refs #53.

test/Jamfile

@@ -143,6 +143,7 @@ run mp_max_element_q.cpp ;
 run mp_nth_element.cpp ;
 run mp_nth_element_q.cpp ;
 run mp_back.cpp ;
+run mp_back_2.cpp ;
 run mp_pop_back.cpp ;
 run mp_flatten.cpp ;
 run mp_rotate_left.cpp ;

test/mp_back_2.cpp

@@ -0,0 +1,57 @@
+// Copyright 2023 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/mp11/algorithm.hpp>
+#include <boost/mp11/list.hpp>
+#include <boost/mp11/utility.hpp>
+#include <boost/mp11/integral.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/core/lightweight_test_trait.hpp>
+
+template<auto... A> struct V1 {};
+template<int... I> struct V2 {};
+
+int main()
+{
+    using boost::mp11::mp_back;
+    using boost::mp11::mp_false;
+    using boost::mp11::mp_int;
+
+    using L1 = V1<false>;
+    BOOST_TEST_TRAIT_SAME(mp_back<L1>, mp_false);
+
+    using L2 = V1<false, true, 4>;
+    BOOST_TEST_TRAIT_SAME(mp_back<L2>, mp_int<4>);
+
+    using L3 = V2<0>;
+    BOOST_TEST_TRAIT_SAME(mp_back<L3>, mp_int<0>);
+
+    using L4 = V2<0, 1, 2, 3, 4, 5, 6, 7, 8, 9>;
+    BOOST_TEST_TRAIT_SAME(mp_back<L4>, mp_int<9>);
+
+    using boost::mp11::mp_iota_c;
+    using boost::mp11::mp_rename_v;
+    using boost::mp11::mp_size_t;
+
+    int const N = 137;
+
+    using L6 = mp_rename_v<mp_iota_c<N>, V1>;
+    BOOST_TEST_TRAIT_SAME(mp_back<L6>, mp_size_t<N-1>);
+
+    using boost::mp11::mp_valid;
+
+    using L7 = V1<>;
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_back, L7>));
+
+    return boost::report_errors();
+}
+
+#endif