libstdc++: Fix std::get<T> for std::pair with reference members [PR121745]

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.

It might appear that std::move(p).first would also work, but the
language rules say that for std::pair<T&&, U> that would produce T&
rather than the expected T&& (see the discussion in P2445R1 §8.2).

Additional tests are added to verify all combinations of reference
members, value categories, and const-qualification.

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 all value
	categories and cv-qualification.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

2 files changed, 56 insertions, 4 deletions

diff --git a/libstdc++-v3/include/bits/stl_pair.h b/libstdc++-v3/include/bits/stl_pair.h
index 2de7439..231d0bb 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 33ebf7a..05a61c3 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 +33,55 @@ 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));
+}
+
+template<typename T, typename Pair>
+using get_t = decltype(std::get<T>(std::declval<Pair>()));
+
+// Check that get<T>(Pair) returns Ret
+template<typename T, typename Pair, typename Ret>
+constexpr bool verify = std::is_same<get_t<T, Pair>, Ret>::value;
+
+template<typename T1, typename T2>
+void
+check()
+{
+  // Overloads for accessing first member
+  static_assert( verify<T1, std::pair<T1, T2>&, T1&>,
+		 "T1& get(pair<T1, T2>&)" );
+  static_assert( verify<T1, const std::pair<T1, T2>&, const T1&>,
+		 "const T1& get(const pair<T1, T2>&)" );
+  static_assert( verify<T1, std::pair<T1, T2>&&, T1&&>,
+		 "T1&& get(pair<T1, T2>&&)" );
+  static_assert( verify<T1, const std::pair<T1, T2>&&, const T1&&>,
+		 "const T1&& get(const pair<T1, T2>&&)" );
+
+  // Overloads for accessing second member
+  static_assert( verify<T2, std::pair<T1, T2>&, T2&>,
+		 "T2& get(pair<T1, T2>&)" );
+  static_assert( verify<T2, const std::pair<T1, T2>&, const T2&>,
+		 "const T2& get(const pair<T1, T2>&)" );
+  static_assert( verify<T2, std::pair<T1, T2>&&, T2&&>,
+		 "T2&& get(pair<T1, T2>&&)" );
+  static_assert( verify<T2, const std::pair<T1, T2>&&, const T2&&>,
+		 "const T2&& get(const pair<T1, T2>&&)" );
+}
+
+void
+test_all()
+{
+  check<float, int>();
+  check<float&, int&>();
+  check<float&&, int&&>();
+}