From f6d92069a9f75f5211fc0b44ded2d2241f9e2d38 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@pdimov.com>
Date: Mon, 29 May 2017 05:56:55 +0300
Subject: [PATCH] Make mp_at SFINAE-friendly

---
 include/boost/mp11/algorithm.hpp |  4 ++--
 test/Jamfile.v2                  |  1 +
 test/mp_at_sf.cpp                | 39 ++++++++++++++++++++++++++++++++
 3 files changed, 42 insertions(+), 2 deletions(-)
 create mode 100644 test/mp_at_sf.cpp

diff --git a/include/boost/mp11/algorithm.hpp b/include/boost/mp11/algorithm.hpp
index d33b248..5d27cba 100644
--- a/include/boost/mp11/algorithm.hpp
+++ b/include/boost/mp11/algorithm.hpp
@@ -325,8 +325,8 @@ template<class L, std::size_t I> struct mp_at_c_impl
 
 } // namespace detail
 
-template<class L, std::size_t I> using mp_at_c = typename detail::mp_at_c_impl<L, I>::type;
-template<class L, class I> using mp_at = typename detail::mp_at_c_impl<L, std::size_t{ I::value }>::type;
+template<class L, std::size_t I> using mp_at_c = typename mp_if_c<(I < mp_size<L>::value), detail::mp_at_c_impl<L, I>, void>::type;
+template<class L, class I> using mp_at = mp_at_c<L, std::size_t{ I::value }>;
 
 // mp_take(_c)<L, N>
 namespace detail
diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index 2d6df0a..b086740 100644
--- a/test/Jamfile.v2
+++ b/test/Jamfile.v2
@@ -48,6 +48,7 @@ run mp_product.cpp : : : $(REQ) ;
 run mp_drop.cpp : : : $(REQ) ;
 run mp_iota.cpp : : : $(REQ) ;
 run mp_at.cpp : : : $(REQ) ;
+run mp_at_sf.cpp : : : $(REQ) ;
 run mp_take.cpp : : : $(REQ) ;
 run mp_replace.cpp : : : $(REQ) ;
 run mp_replace_if.cpp : : : $(REQ) ;
diff --git a/test/mp_at_sf.cpp b/test/mp_at_sf.cpp
new file mode 100644
index 0000000..00463f9
--- /dev/null
+++ b/test/mp_at_sf.cpp
@@ -0,0 +1,39 @@
+
+// Copyright 2015, 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/utility.hpp>
+#include <boost/mp11/algorithm.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <type_traits>
+
+using boost::mp11::mp_valid;
+using boost::mp11::mp_at;
+using boost::mp11::mp_size_t;
+using boost::mp11::mp_list;
+
+int main()
+{
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_at>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_at, void>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_at, void, void>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_at, void, void, void>));
+
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_at, void, mp_size_t<0>>));
+
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_at, mp_list<>, mp_size_t<0>>));
+
+    BOOST_TEST_TRAIT_TRUE((mp_valid<mp_at, mp_list<void>, mp_size_t<0>>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_at, mp_list<void>, mp_size_t<1>>));
+
+    BOOST_TEST_TRAIT_TRUE((mp_valid<mp_at, mp_list<void, void>, mp_size_t<0>>));
+    BOOST_TEST_TRAIT_TRUE((mp_valid<mp_at, mp_list<void, void>, mp_size_t<1>>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_at, mp_list<void, void>, mp_size_t<2>>));
+
+    return boost::report_errors();
+}