Add mp_eval_or

test/mp_eval_or.cpp

@@ -0,0 +1,51 @@
+
+// Copyright 2019 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/utility.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <cstddef>
+
+template<class T> using difference_type = typename T::difference_type;
+
+struct X
+{
+};
+
+struct Y
+{
+    using difference_type = int;
+};
+
+int main()
+{
+    using boost::mp11::mp_eval_or;
+    using boost::mp11::mp_eval_or_q;
+    using boost::mp11::mp_identity;
+    using boost::mp11::mp_quote;
+
+    BOOST_TEST_TRAIT_SAME(mp_eval_or<void, mp_identity>, void);
+    BOOST_TEST_TRAIT_SAME(mp_eval_or<void, mp_identity, int>, mp_identity<int>);
+    BOOST_TEST_TRAIT_SAME(mp_eval_or<void, mp_identity, int, int>, void);
+
+    using Q_identity = mp_quote<mp_identity>;
+
+    BOOST_TEST_TRAIT_SAME(mp_eval_or_q<void, Q_identity>, void);
+    BOOST_TEST_TRAIT_SAME(mp_eval_or_q<void, Q_identity, int>, mp_identity<int>);
+    BOOST_TEST_TRAIT_SAME(mp_eval_or_q<void, Q_identity, int, int>, void);
+
+    BOOST_TEST_TRAIT_SAME(mp_eval_or<std::ptrdiff_t, difference_type, X>, std::ptrdiff_t);
+    BOOST_TEST_TRAIT_SAME(mp_eval_or<std::ptrdiff_t, difference_type, Y>, int);
+
+    using Q_diff_type = mp_quote<difference_type>;
+
+    BOOST_TEST_TRAIT_SAME(mp_eval_or_q<std::ptrdiff_t, Q_diff_type, X>, std::ptrdiff_t);
+    BOOST_TEST_TRAIT_SAME(mp_eval_or_q<std::ptrdiff_t, Q_diff_type, Y>, int);
+
+    return boost::report_errors();
+}