From fc8bfcf601c35b0cc4e337be2f59f2a16912af5e Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Thu, 14 Feb 2019 21:03:13 +0200
Subject: [PATCH] Add mp_eval_or

---
 include/boost/mp11/utility.hpp |  4 +++
 test/Jamfile                   |  1 +
 test/mp_eval_or.cpp            | 51 ++++++++++++++++++++++++++++++++++
 3 files changed, 56 insertions(+)
 create mode 100644 test/mp_eval_or.cpp

diff --git a/include/boost/mp11/utility.hpp b/include/boost/mp11/utility.hpp
index b085d0e..38989f4 100644
--- a/include/boost/mp11/utility.hpp
+++ b/include/boost/mp11/utility.hpp
@@ -153,6 +153,10 @@ template<class C, class T, class Q, class... U> using mp_eval_if_q = typename de
 template<class C, class T, template<class...> class F, class... U> using mp_eval_if_not = mp_eval_if<mp_not<C>, T, F, U...>;
 template<class C, class T, class Q, class... U> using mp_eval_if_not_q = mp_eval_if<mp_not<C>, T, Q::template fn, U...>;
 
+// mp_eval_or
+template<class T, template<class...> class F, class... U> using mp_eval_or = mp_eval_if_not<mp_valid<F, U...>, T, F, U...>;
+template<class T, class Q, class... U> using mp_eval_or_q = mp_eval_or<T, Q::template fn, U...>;
+
 // mp_cond
 
 // so elegant; so doesn't work
diff --git a/test/Jamfile b/test/Jamfile
index 437c187..74f6b67 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -125,6 +125,7 @@ run mp_cond.cpp ;
 run mp_cond_sf.cpp ;
 run mp_not_fn.cpp ;
 run mp_eval_if_not.cpp ;
+run mp_eval_or.cpp ;
 
 # integer_sequence
 run integer_sequence.cpp ;
diff --git a/test/mp_eval_or.cpp b/test/mp_eval_or.cpp
new file mode 100644
index 0000000..c56a91b
--- /dev/null
+++ b/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();
+}