[pair] Support reference_wrappers in make_pair

Author: ldionne

include/boost/hana/fwd/pair.hpp

@@ -105,6 +105,13 @@ namespace boost { namespace hana {
     //! @relates hana::pair
     //!
     //!
+    //! @note
+    //! `make<pair_tag>` supports reference wrappers. When a reference wrapper
+    //! is passed to it, the resulting `hana::pair` will hold a reference
+    //! to the object enclosed in the reference wrapper instead of the object
+    //! itself.
+    //!
+    //!
     //! Example
     //! -------
     //! @include example/pair/make.cpp

include/boost/hana/pair.hpp

@@ -13,6 +13,7 @@ Distributed under the Boost Software License, Version 1.0.
 #include <boost/hana/fwd/pair.hpp>
 
 #include <boost/hana/basic_tuple.hpp>
+#include <boost/hana/detail/as_container_element.hpp>
 #include <boost/hana/detail/intrinsics.hpp>
 #include <boost/hana/detail/operators/adl.hpp>
 #include <boost/hana/detail/operators/comparable.hpp>
@@ -120,8 +121,8 @@ namespace boost { namespace hana {
     struct make_impl<pair_tag> {
         template <typename F, typename S>
         static constexpr pair<
-            typename std::decay<F>::type,
-            typename std::decay<S>::type
+            detail::as_container_element_t<F>,
+            detail::as_container_element_t<S>
         > apply(F&& f, S&& s) {
             return {static_cast<F&&>(f), static_cast<S&&>(s)};
         }

test/pair/make.ref.cpp

@@ -0,0 +1,30 @@
+/*
+@copyright Louis Dionne 2015
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+ */
+
+#include <boost/hana/assert.hpp>
+#include <boost/hana/first.hpp>
+#include <boost/hana/pair.hpp>
+#include <boost/hana/second.hpp>
+
+#include <functional>
+namespace hana = boost::hana;
+
+
+int main() {
+    int i = 0;
+    char c = 'x';
+    hana::pair<int&, char const&> refs = hana::make_pair(std::ref(i), std::cref(c));
+
+    int& i_ref = hana::first(refs);
+    BOOST_HANA_RUNTIME_CHECK(&i_ref == &i);
+
+    i_ref = 3;
+    BOOST_HANA_RUNTIME_CHECK(i == 3);
+
+    char const& c_ref = hana::second(refs);
+    BOOST_HANA_RUNTIME_CHECK(&c_ref == &c);
+    BOOST_HANA_RUNTIME_CHECK(c == 'x');
+}