libstdc++: Fix fallout from P2321R2 std::pair/tuple enhancements

r13-2159-g72886fcc626953 caused some testsuite regressions in C++23 mode:

  FAIL: 20_util/pair/requirements/explicit_instantiation/1.cc (test for excess errors)
  FAIL: 20_util/tuple/53648.cc (test for excess errors)
  FAIL: 20_util/tuple/cons/noexcept_specs.cc (test for excess errors)
  FAIL: 20_util/tuple/requirements/explicit_instantiation.cc (test for excess errors)

The test noexcept_specs.cc just needs to be updated to consider the
additional converting constructors of tuple in C++23 mode, which happen
to improve constructing from a const tuple rvalue that has an rvalue
reference element (for the better, as far as I can tell).

The other three tests fail because they explicitly instantiate a
specialization of pair/tuple whose elements are not all const swappable,
which in C++23 mode now results in a hard error due to the new const
swap member function.  Rather than XFAILing the tests in C++23 mode,
this patch adds non-standard constraints to this member function so that
we continue to accept such explicit instantiations.

libstdc++-v3/ChangeLog:

	* include/bits/stl_pair.h (pair::swap const): Add non-standard
	is_swappable_v constraints.
	* include/std/tuple (tuple::swap const): Likewise.
	* testsuite/20_util/tuple/cons/noexcept_specs.cc: Correct some
	asserts in C++23 mode.

3 files changed, 56 insertions, 0 deletions

diff --git a/libstdc++-v3/include/bits/stl_pair.h b/libstdc++-v3/include/bits/stl_pair.h
index bffca0d..d0f07b0 100644
--- a/libstdc++-v3/include/bits/stl_pair.h
+++ b/libstdc++-v3/include/bits/stl_pair.h
@@ -213,10 +213,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 
 #if __cplusplus > 202002L
+      // As an extension, we constrain the const swap member function in order
+      // to continue accepting explicit instantiation of pairs whose elements
+      // are not all const swappable.  Without this constraint, such an
+      // explicit instantiation would also instantiate the ill-formed body of
+      // this function and yield a hard error.  This constraint shouldn't
+      // affect the behavior of valid programs.
       constexpr void
       swap(const pair& __p) const
       noexcept(__and_v<__is_nothrow_swappable<const _T1>,
 		       __is_nothrow_swappable<const _T2>>)
+      requires is_swappable_v<const _T1> && is_swappable_v<const _T2>
       {
 	using std::swap;
 	swap(first, __p.first);
diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index 05433d5..ddd7c22 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -1176,9 +1176,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { _Inherited::_M_swap(__in); }
 
 #if __cplusplus > 202002L
+      // As an extension, we constrain the const swap member function in order
+      // to continue accepting explicit instantiation of tuples whose elements
+      // are not all const swappable.  Without this constraint, such an
+      // explicit instantiation would also instantiate the ill-formed body of
+      // this function and yield a hard error.  This constraint shouldn't
+      // affect the behavior of valid programs.
       constexpr void
       swap(const tuple& __in) const
       noexcept(__and_v<__is_nothrow_swappable<const _Elements>...>)
+      requires (is_swappable_v<const _Elements> && ...)
       { _Inherited::_M_swap(__in); }
 #endif // C++23
     };
@@ -1730,6 +1737,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       swap(const tuple& __in) const
       noexcept(__and_v<__is_nothrow_swappable<const _T1>,
 		       __is_nothrow_swappable<const _T2>>)
+      requires is_swappable_v<const _T1> && is_swappable_v<const _T2>
       { _Inherited::_M_swap(__in); }
 #endif // C++23
     };
diff --git a/libstdc++-v3/testsuite/20_util/tuple/cons/noexcept_specs.cc b/libstdc++-v3/testsuite/20_util/tuple/cons/noexcept_specs.cc
index 6044a37..a326d1b 100644
--- a/libstdc++-v3/testsuite/20_util/tuple/cons/noexcept_specs.cc
+++ b/libstdc++-v3/testsuite/20_util/tuple/cons/noexcept_specs.cc
@@ -503,7 +503,17 @@ namespace NothrowCopyThrowMoveThrowCopyConversionNothrowMoveConversion
   static_assert(!std::is_nothrow_constructible<BT, IT&>::value, "");
   static_assert(!std::is_nothrow_constructible<BT, const IT &>::value, "");
   static_assert(!std::is_nothrow_constructible<BT, std::tuple<int&>>::value, "");
+#if __cplusplus > 202002L
+  // C++23 extended tuple's constructor overload set as part of P2321R2, after
+  // which its converting constructors more accurately forward the elements
+  // from a non-const tuple lvalue and from a const tuple rvalue.  In particular
+  // for the below test we now forward int&& as an rvalue reference instead of
+  // as an lvalue reference, which means we now select the noexcept B(int&&)
+  // ctor instead of the non-noexcept B(const int&) ctor.
+  static_assert(std::is_nothrow_constructible<BT, const std::tuple<int&&>>::value, "");
+#else
   static_assert(!std::is_nothrow_constructible<BT, const std::tuple<int&&>>::value, "");
+#endif
 
   static_assert(test_trait::is_nothrow_convertible<int,BT>::value,"");
   static_assert(!test_trait::is_nothrow_convertible<const int,BT>::value,"");
@@ -515,7 +525,13 @@ namespace NothrowCopyThrowMoveThrowCopyConversionNothrowMoveConversion
   static_assert(!test_trait::is_nothrow_convertible<IT&,BT>::value,"");
   static_assert(!test_trait::is_nothrow_convertible<const IT &,BT>::value,"");
   static_assert(!test_trait::is_nothrow_convertible<std::tuple<int&>,BT>::value,"");
+#if __cplusplus > 202002L
+  // See the note about P2321R2 above.
+  static_assert(test_trait::is_nothrow_convertible<const std::tuple<int&&>,BT>::value,"");
+#else
   static_assert(!test_trait::is_nothrow_convertible<const std::tuple<int&&>,BT>::value,"");
+#endif
+
 
 
   static_assert(!std::is_nothrow_constructible<BT, B>::value, "");
@@ -528,7 +544,12 @@ namespace NothrowCopyThrowMoveThrowCopyConversionNothrowMoveConversion
   static_assert(std::is_nothrow_constructible<BT, BT&>::value, "");
   static_assert(std::is_nothrow_constructible<BT, const BT &>::value, "");
   static_assert(std::is_nothrow_constructible<BT, std::tuple<B&>>::value, "");
+#if __cplusplus > 202002L
+  // See the note about P2321R2 above.
+  static_assert(!std::is_nothrow_constructible<BT, const std::tuple<B&&>>::value, "");
+#else
   static_assert(std::is_nothrow_constructible<BT, const std::tuple<B&&>>::value, "");
+#endif
 
   static_assert(!test_trait::is_nothrow_convertible<B,BT>::value,"");
   static_assert(test_trait::is_nothrow_convertible<const B,BT>::value,"");
@@ -540,7 +561,12 @@ namespace NothrowCopyThrowMoveThrowCopyConversionNothrowMoveConversion
   static_assert(test_trait::is_nothrow_convertible<BT&,BT>::value,"");
   static_assert(test_trait::is_nothrow_convertible<const BT &,BT>::value,"");
   static_assert(test_trait::is_nothrow_convertible<std::tuple<B&>,BT>::value,"");
+#if __cplusplus > 202002L
+  // See the note about P2321R2 above.
+  static_assert(!test_trait::is_nothrow_convertible<const std::tuple<B&&>,BT>::value,"");
+#else
   static_assert(test_trait::is_nothrow_convertible<const std::tuple<B&&>,BT>::value,"");
+#endif
 
 /* explicit */
   static_assert(std::is_nothrow_constructible<DT, int>::value, "");
@@ -553,7 +579,12 @@ namespace NothrowCopyThrowMoveThrowCopyConversionNothrowMoveConversion
   static_assert(!std::is_nothrow_constructible<DT, IT&>::value, "");
   static_assert(!std::is_nothrow_constructible<DT, const IT &>::value, "");
   static_assert(!std::is_nothrow_constructible<DT, std::tuple<int&>>::value, "");
+#if __cplusplus > 202002L
+  // See the note about P2321R2 above.
+  static_assert(std::is_nothrow_constructible<DT, const std::tuple<int&&>>::value, "");
+#else
   static_assert(!std::is_nothrow_constructible<DT, const std::tuple<int&&>>::value, "");
+#endif
 
   static_assert(!std::is_nothrow_constructible<DT, D>::value, "");
   static_assert(std::is_nothrow_constructible<DT,const D>::value, "");
@@ -565,7 +596,12 @@ namespace NothrowCopyThrowMoveThrowCopyConversionNothrowMoveConversion
   static_assert(std::is_nothrow_constructible<DT, DT&>::value, "");
   static_assert(std::is_nothrow_constructible<DT, const DT &>::value, "");
   static_assert(std::is_nothrow_constructible<DT, std::tuple<D&>>::value, "");
+#if __cplusplus > 202002L
+  // See note about P2321R2 above.
+  static_assert(!std::is_nothrow_constructible<DT, const std::tuple<D&&>>::value, "");
+#else
   static_assert(std::is_nothrow_constructible<DT, const std::tuple<D&&>>::value, "");
+#endif
 
   static_assert(!test_trait::is_nothrow_convertible<DT,DT>::value,"");
   static_assert(test_trait::is_nothrow_convertible<const DT,DT>::value,"");
@@ -884,7 +920,12 @@ namespace ThrowMoveNothrowConversion
   static_assert(std::is_nothrow_constructible<DT, IT&>::value, "");
   static_assert(std::is_nothrow_constructible<DT, const IT &>::value, "");
   static_assert(std::is_nothrow_constructible<DT, std::tuple<int&>>::value, "");
+#if __cplusplus > 202002L
+  // See the note about P2321R2 above.
+  static_assert(!std::is_nothrow_constructible<DT, const std::tuple<int&&>>::value, "");
+#else
   static_assert(std::is_nothrow_constructible<DT, const std::tuple<int&&>>::value, "");
+#endif
 
   static_assert(test_trait::is_nothrow_convertible<DT,DT>::value,"");
   static_assert(test_trait::is_nothrow_convertible<D,DT>::value,"");