[bugfix] Fix incorrect result for first and second in nested pair cases

Author: ldionne

include/boost/hana/pair.hpp

@@ -161,16 +161,46 @@ BOOST_HANA_NAMESPACE_BEGIN
 
     template <>
     struct first_impl<pair_tag> {
-        template <typename P>
-        static constexpr decltype(auto) apply(P&& p)
-        { return detail::ebo_get<detail::pix<0>>(static_cast<P&&>(p)); }
+        template <typename First, typename Second>
+        static constexpr decltype(auto) apply(hana::pair<First, Second>& p) {
+            return detail::ebo_get<detail::pix<0>>(
+                static_cast<detail::ebo<detail::pix<0>, First>&>(p)
+            );
+        }
+        template <typename First, typename Second>
+        static constexpr decltype(auto) apply(hana::pair<First, Second> const& p) {
+            return detail::ebo_get<detail::pix<0>>(
+                static_cast<detail::ebo<detail::pix<0>, First> const&>(p)
+            );
+        }
+        template <typename First, typename Second>
+        static constexpr decltype(auto) apply(hana::pair<First, Second>&& p) {
+            return detail::ebo_get<detail::pix<0>>(
+                static_cast<detail::ebo<detail::pix<0>, First>&&>(p)
+            );
+        }
     };
 
     template <>
     struct second_impl<pair_tag> {
-        template <typename P>
-        static constexpr decltype(auto) apply(P&& p)
-        { return detail::ebo_get<detail::pix<1>>(static_cast<P&&>(p)); }
+        template <typename First, typename Second>
+        static constexpr decltype(auto) apply(hana::pair<First, Second>& p) {
+            return detail::ebo_get<detail::pix<1>>(
+                static_cast<detail::ebo<detail::pix<1>, Second>&>(p)
+            );
+        }
+        template <typename First, typename Second>
+        static constexpr decltype(auto) apply(hana::pair<First, Second> const& p) {
+            return detail::ebo_get<detail::pix<1>>(
+                static_cast<detail::ebo<detail::pix<1>, Second> const&>(p)
+            );
+        }
+        template <typename First, typename Second>
+        static constexpr decltype(auto) apply(hana::pair<First, Second>&& p) {
+            return detail::ebo_get<detail::pix<1>>(
+                static_cast<detail::ebo<detail::pix<1>, Second>&&>(p)
+            );
+        }
     };
 BOOST_HANA_NAMESPACE_END
 

test/issues/github_331.cpp

@@ -0,0 +1,45 @@
+// Copyright Louis Dionne 2013-2017
+// 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/at.hpp>
+#include <boost/hana/first.hpp>
+#include <boost/hana/integral_constant.hpp>
+#include <boost/hana/pair.hpp>
+#include <boost/hana/second.hpp>
+#include <boost/hana/tuple.hpp>
+
+#include <type_traits>
+namespace hana = boost::hana;
+
+
+// In GitHub issue #331, we noticed that `first` and `second` could sometimes
+// return the wrong member in case of nested pairs. This is due to the way we
+// inherit from base classes to enable EBO. We also check for `basic_tuple`,
+// because both are implemented similarly.
+
+int main() {
+    {
+        using Nested = hana::pair<hana::int_<1>, hana::int_<2>>;
+        using Pair = hana::pair<hana::int_<0>, Nested>;
+        Pair pair{};
+
+        auto a = hana::first(pair);
+        static_assert(std::is_same<decltype(a), hana::int_<0>>{}, "");
+
+        auto b = hana::second(pair);
+        static_assert(std::is_same<decltype(b), Nested>{}, "");
+    }
+
+    {
+        using Nested = hana::basic_tuple<hana::int_<1>, hana::int_<2>>;
+        using Tuple = hana::basic_tuple<hana::int_<0>, Nested>;
+        Tuple tuple{};
+
+        auto a = hana::at_c<0>(tuple);
+        static_assert(std::is_same<decltype(a), hana::int_<0>>{}, "");
+
+        auto b = hana::at_c<1>(tuple);
+        static_assert(std::is_same<decltype(b), Nested>{}, "");
+    }
+}