[optional] Add support for reference_wrapper in just()

Author: ldionne

include/boost/hana/fwd/optional.hpp

@@ -147,6 +147,11 @@ namespace boost { namespace hana {
     //! `make<optional_tag>(x)` is equivalent to `just(x)`. This is provided
     //! for consistency with the other `make<...>` functions.
     //!
+    //! @note
+    //! `make<optional_tag>` supports reference wrappers. When a reference
+    //! wrapper is passed to it, the resulting `hana::optional` will hold a
+    //! reference to the object instead of the object itself.
+    //!
     //!
     //! Example
     //! -------

include/boost/hana/optional.hpp

@@ -14,6 +14,7 @@ Distributed under the Boost Software License, Version 1.0.
 
 #include <boost/hana/bool.hpp>
 #include <boost/hana/core/tag_of.hpp>
+#include <boost/hana/detail/as_container_element.hpp>
 #include <boost/hana/detail/operators/adl.hpp>
 #include <boost/hana/detail/operators/comparable.hpp>
 #include <boost/hana/detail/operators/monad.hpp>
@@ -72,14 +73,14 @@ namespace boost { namespace hana {
         constexpr T const& operator*() const& { return this->val; }
         constexpr T operator*() && { return std::move(this->val); }
 
-        constexpr T* operator->() & { return &this->val; }
-        constexpr T const* operator->() const& { return &this->val; }
+        constexpr auto operator->() & { return &this->val; }
+        constexpr auto operator->() const& { return &this->val; }
     };
 
     //! @cond
     template <typename T>
     constexpr auto make_just_t::operator()(T&& t) const {
-        return optional<typename std::decay<T>::type>(
+        return optional<detail::as_container_element_t<T>>(
             static_cast<T&&>(t)
         );
     }

test/optional/make.ref.cpp

@@ -0,0 +1,34 @@
+/*
+@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/optional.hpp>
+
+#include <functional>
+namespace hana = boost::hana;
+
+
+int main() {
+    {
+        int i = 0;
+        auto ref = hana::just(std::ref(i));
+
+        int& i_ref = *ref;
+        BOOST_HANA_RUNTIME_CHECK(&i_ref == &i);
+
+        i_ref = 3;
+        BOOST_HANA_RUNTIME_CHECK(i == 3);
+    }
+
+    {
+        int const i = 4;
+        auto ref = hana::just(std::cref(i));
+
+        int const& i_ref = *ref;
+        BOOST_HANA_RUNTIME_CHECK(&i_ref == &i);
+        BOOST_HANA_RUNTIME_CHECK(i == 4);
+    }
+}