[PATCH] libstdc++: Fix std::get for std::pair with reference members [PR121745]

[Jonathan Wakely]

Make the std::get<T> overloads for rvalues use std::forward<T>(p.first)
not std::move(p.first), so that lvalue reference members are not
incorrectly converted to rvalues.

libstdc++-v3/ChangeLog:

        PR libstdc++/121745
        * include/bits/stl_pair.h (get): Use forward instead of move in
        std::get<T> overloads for rvalue pairs.
        * testsuite/20_util/pair/astuple/get_by_type.cc: Check rvalue
        arguments with reference members.
---

Tested powerpc64-linux. We should backport this too.

 libstdc++-v3/include/bits/stl_pair.h                 |  8 ++++----
 .../testsuite/20_util/pair/astuple/get_by_type.cc    | 12 ++++++++++++
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/stl_pair.h 
b/libstdc++-v3/include/bits/stl_pair.h
index 393f6a016196..661335b466a3 100644
--- a/libstdc++-v3/include/bits/stl_pair.h
+++ b/libstdc++-v3/include/bits/stl_pair.h
@@ -1315,12 +1315,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template <typename _Tp, typename _Up>
     constexpr _Tp&&
     get(pair<_Tp, _Up>&& __p) noexcept
-    { return std::move(__p.first); }
+    { return std::forward<_Tp>(__p.first); }
 
   template <typename _Tp, typename _Up>
     constexpr const _Tp&&
     get(const pair<_Tp, _Up>&& __p) noexcept
-    { return std::move(__p.first); }
+    { return std::forward<const _Tp>(__p.first); }
 
   template <typename _Tp, typename _Up>
     constexpr _Tp&
@@ -1335,12 +1335,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template <typename _Tp, typename _Up>
     constexpr _Tp&&
     get(pair<_Up, _Tp>&& __p) noexcept
-    { return std::move(__p.second); }
+    { return std::forward<_Tp>(__p.second); }
 
   template <typename _Tp, typename _Up>
     constexpr const _Tp&&
     get(const pair<_Up, _Tp>&& __p) noexcept
-    { return std::move(__p.second); }
+    { return std::forward<const _Tp>(__p.second); }
 #endif // __glibcxx_tuples_by_type
 
 
diff --git a/libstdc++-v3/testsuite/20_util/pair/astuple/get_by_type.cc 
b/libstdc++-v3/testsuite/20_util/pair/astuple/get_by_type.cc
index 33ebf7a46b90..9d934db0b69f 100644
--- a/libstdc++-v3/testsuite/20_util/pair/astuple/get_by_type.cc
+++ b/libstdc++-v3/testsuite/20_util/pair/astuple/get_by_type.cc
@@ -33,3 +33,15 @@ void test01()
   const int&&  cpsecond __attribute__((unused)) =
     std::get<int>(std::move(cp));
 }
+
+// PR libstdc++/121745 return of get(pair<_Up, _Tp>&& __p) may be ill-formed
+void
+test_pr121745(std::pair<float&, int&> p)
+{
+  float& pfirst = std::get<float&>(std::move(p));
+  int& psecond  = std::get<int&>(std::move(p));
+
+  const auto& p2 = p;
+  float& p2first = std::get<float&>(std::move(p2));
+  int& p2second  = std::get<int&>(std::move(p2));
+}
-- 
2.51.0