[gcc r15-10284] libstdc++: Fix std::get for std::pair with reference members [PR121745]

[Jonathan Wakely via Libstdc++-cvs]

https://gcc.gnu.org/g:46cfa36e295943d4bfa01c033a17f48c70154eeb

commit r15-10284-g46cfa36e295943d4bfa01c033a17f48c70154eeb
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Mon Sep 1 18:12:27 2025 +0100

    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 <tkami...@redhat.com>
    (cherry picked from commit c8a24f60b6874fca4fb3adb153f8d5f1dd72b951)

Diff:
---
 libstdc++-v3/include/bits/stl_pair.h               |  8 ++--
 .../testsuite/20_util/pair/astuple/get_by_type.cc  | 52 ++++++++++++++++++++++
 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 8c57712b4617..37547ceac62d 100644
--- a/libstdc++-v3/include/bits/stl_pair.h
+++ b/libstdc++-v3/include/bits/stl_pair.h
@@ -1313,12 +1313,12 @@ _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&
@@ -1333,12 +1333,12 @@ _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..05a61c3f302c 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&&>();
+}