[gcc r16-2242] libstdc++: Add comments to deleted std::swap overloads for LWG 2766

[Jonathan Wakely]

https://gcc.gnu.org/g:9840a1db02cb96011f90b8ee86ca2d23b41781db

commit r16-2242-g9840a1db02cb96011f90b8ee86ca2d23b41781db
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Sun Jul 13 15:34:15 2025 +0100

    libstdc++: Add comments to deleted std::swap overloads for LWG 2766
    
    We pre-emptively implemented part of LWG 2766, which still hasn't been
    approved. Add comments to the deleted swap overloads saying why they're
    there, because the standard doesn't require them.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/stl_pair.h (swap): Add comment to deleted
            overload.
            * include/bits/unique_ptr.h (swap): Likewise.
            * include/std/array (swap): Likewise.
            * include/std/optional (swap): Likewise.
            * include/std/tuple (swap): Likewise.
            * include/std/variant (swap): Likewise.
            * testsuite/23_containers/array/tuple_interface/get_neg.cc:
            Adjust dg-error line numbers.

Diff:
---
 libstdc++-v3/include/bits/stl_pair.h                                | 2 ++
 libstdc++-v3/include/bits/unique_ptr.h                              | 2 ++
 libstdc++-v3/include/std/array                                      | 2 ++
 libstdc++-v3/include/std/optional                                   | 2 ++
 libstdc++-v3/include/std/tuple                                      | 4 +++-
 libstdc++-v3/include/std/variant                                    | 2 ++
 .../testsuite/23_containers/array/tuple_interface/get_neg.cc        | 6 +++---
 7 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 8c57712b4617..393f6a016196 100644
--- a/libstdc++-v3/include/bits/stl_pair.h
+++ b/libstdc++-v3/include/bits/stl_pair.h
@@ -1132,6 +1132,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif // C++23
 
 #if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 2766. Swapping non-swappable types
   template<typename _T1, typename _T2>
     typename enable_if<!__and_<__is_swappable<_T1>,
 			       __is_swappable<_T2>>::value>::type
diff --git a/libstdc++-v3/include/bits/unique_ptr.h b/libstdc++-v3/include/bits/unique_ptr.h
index 6ae46a93800c..d76ad63ba7bf 100644
--- a/libstdc++-v3/include/bits/unique_ptr.h
+++ b/libstdc++-v3/include/bits/unique_ptr.h
@@ -832,6 +832,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { __x.swap(__y); }
 
 #if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 2766. Swapping non-swappable types
   template<typename _Tp, typename _Dp>
     typename enable_if<!__is_swappable<_Dp>::value>::type
     swap(unique_ptr<_Tp, _Dp>&,
diff --git a/libstdc++-v3/include/std/array b/libstdc++-v3/include/std/array
index fdcf0b073762..12f010921db1 100644
--- a/libstdc++-v3/include/std/array
+++ b/libstdc++-v3/include/std/array
@@ -381,6 +381,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { __one.swap(__two); }
 
 #if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 2766. Swapping non-swappable types
   template<typename _Tp, std::size_t _Nm>
     __enable_if_t<!__array_traits<_Tp, _Nm>::_Is_swappable::value>
     swap(array<_Tp, _Nm>&, array<_Tp, _Nm>&) = delete;
diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index cc7af5bbd7d2..e5051d72c828 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -1740,6 +1740,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     noexcept(noexcept(__lhs.swap(__rhs)))
     { __lhs.swap(__rhs); }
 
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 2766. Swapping non-swappable types
   template<typename _Tp>
     enable_if_t<!(is_move_constructible_v<_Tp> && is_swappable_v<_Tp>)>
     swap(optional<_Tp>&, optional<_Tp>&) = delete;
diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index b39ce710984c..2e6499eab22d 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -2835,6 +2835,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { __x.swap(__y); }
 
 #if __cpp_lib_ranges_zip // >= C++23
+  /// Exchange the values of two const tuples (if const elements can be swapped)
   template<typename... _Elements>
     requires (is_swappable_v<const _Elements> && ...)
     constexpr void
@@ -2844,7 +2845,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif // C++23
 
 #if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
-  /// Exchange the values of two const tuples (if const elements can be swapped)
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 2766. Swapping non-swappable types
   template<typename... _Elements>
     _GLIBCXX20_CONSTEXPR
     typename enable_if<!__and_<__is_swappable<_Elements>...>::value>::type
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index ec46ff1dabb5..2f44f9700283 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1387,6 +1387,8 @@ namespace __detail::__variant
     noexcept(noexcept(__lhs.swap(__rhs)))
     { __lhs.swap(__rhs); }
 
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 2766. Swapping non-swappable types
   template<typename... _Types>
     enable_if_t<!((is_move_constructible_v<_Types> && ...)
 		   && (is_swappable_v<_Types> && ...))>
diff --git a/libstdc++-v3/testsuite/23_containers/array/tuple_interface/get_neg.cc b/libstdc++-v3/testsuite/23_containers/array/tuple_interface/get_neg.cc
index 25511e79941d..e1e9ce9bdac1 100644
--- a/libstdc++-v3/testsuite/23_containers/array/tuple_interface/get_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/array/tuple_interface/get_neg.cc
@@ -26,6 +26,6 @@ int n1 = std::get<1>(a);
 int n2 = std::get<1>(std::move(a));
 int n3 = std::get<1>(ca);
 
-// { dg-error "static assertion failed" "" { target *-*-* } 394 }
-// { dg-error "static assertion failed" "" { target *-*-* } 403 }
-// { dg-error "static assertion failed" "" { target *-*-* } 412 }
+// { dg-error "static assertion failed" "" { target *-*-* } 396 }
+// { dg-error "static assertion failed" "" { target *-*-* } 405 }
+// { dg-error "static assertion failed" "" { target *-*-* } 414 }