Add mp_is_list

doc/mp11/list.adoc

@@ -21,6 +21,12 @@ http://www.boost.org/LICENSE_1_0.txt
 such as `std::tuple` or `std::variant`. Even `std::pair` can be used if the transformation does not alter the number of the elements in
 the list.
 
+## mp_is_list<L>
+
+    template<class L> using mp_is_list = /*...*/;
+
+`mp_is_list<L>` is `mp_true` if `L` is a list (an instantiation of a class template whose template parameters are types), `mp_false` otherwise.
+
 ## mp_size<L>
 
     template<class L> using mp_size = /*...*/;

include/boost/mp11/list.hpp

@@ -19,6 +19,24 @@ namespace boost
 namespace mp11
 {
 
+// mp_is_list<L>
+namespace detail
+{
+
+template<class L> struct mp_is_list_impl
+{
+    using type = mp_false;
+};
+
+template<template<class...> class L, class... T> struct mp_is_list_impl<L<T...>>
+{
+    using type = mp_true;
+};
+
+} // namespace detail
+
+template<class L> using mp_is_list = typename detail::mp_is_list_impl<L>::type;
+
 // mp_size<L>
 namespace detail
 {

test/Jamfile

@@ -30,6 +30,7 @@ run mp_replace_front.cpp ;
 run mp_replace_second.cpp ;
 run mp_replace_third.cpp ;
 run mp_apply_q.cpp ;
+run mp_is_list.cpp ;
 
 # algorithm
 run mp_assign.cpp ;

test/mp_is_list.cpp

@@ -0,0 +1,50 @@
+
+// Copyright 2017 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/list.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <type_traits>
+#include <tuple>
+#include <utility>
+
+int main()
+{
+    using boost::mp11::mp_list;
+    using boost::mp11::mp_is_list;
+    using boost::mp11::mp_true;
+    using boost::mp11::mp_false;
+
+    {
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_is_list<void>, mp_false>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_is_list<int>, mp_false>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_is_list<char[]>, mp_false>));
+    }
+
+    {
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_is_list<mp_list<>>, mp_true>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_is_list<mp_list<void>>, mp_true>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_is_list<mp_list<void, void>>, mp_true>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_is_list<mp_list<void, void, void>>, mp_true>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_is_list<mp_list<void, void, void, void>>, mp_true>));
+    }
+
+    {
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_is_list<std::tuple<>>, mp_true>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_is_list<std::tuple<void>>, mp_true>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_is_list<std::tuple<void, void>>, mp_true>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_is_list<std::tuple<void, void, void>>, mp_true>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_is_list<std::tuple<void, void, void, void>>, mp_true>));
+    }
+
+    {
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_is_list<std::pair<void, void>>, mp_true>));
+    }
+
+    return boost::report_errors();
+}