commit d319517e809ee50496db29e552f86a83a14c837c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Jun 8 14:56:57 2021

    libstdc++: Finish implementing LWG 3413 for propagate_const
    
    We already have conditional noexcept so this just constrains the
    non-member swap overload.
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * include/experimental/propagate_const (swap): Constrain.
            * testsuite/experimental/propagate_const/swap/lwg3413.cc: New test.

diff --git a/libstdc++-v3/include/experimental/propagate_const b/libstdc++-v3/include/experimental/propagate_const
index 0d03c13c5e1..162b4783dd7 100644
--- a/libstdc++-v3/include/experimental/propagate_const
+++ b/libstdc++-v3/include/experimental/propagate_const
@@ -113,6 +113,7 @@ inline namespace fundamentals_v2
       constexpr propagate_const() = default;
       propagate_const(const propagate_const& __p) = delete;
       constexpr propagate_const(propagate_const&& __p) = default;
+
       template <typename _Up, typename
 		enable_if<__and_<is_constructible<_Tp, _Up&&>,
 				 is_convertible<_Up&&, _Tp>>::value, bool
@@ -120,6 +121,7 @@ inline namespace fundamentals_v2
       constexpr propagate_const(propagate_const<_Up>&& __pu)
 	: _M_t(std::move(get_underlying(__pu)))
       {}
+
       template <typename _Up, typename
 		enable_if<__and_<is_constructible<_Tp, _Up&&>,
 				 __not_<is_convertible<_Up&&, _Tp>>>::value,
@@ -127,6 +129,7 @@ inline namespace fundamentals_v2
       constexpr explicit propagate_const(propagate_const<_Up>&& __pu)
 	: _M_t(std::move(get_underlying(__pu)))
       {}
+
       template <typename _Up, typename
 		enable_if<__and_<is_constructible<_Tp, _Up&&>,
 				 is_convertible<_Up&&, _Tp>,
@@ -136,6 +139,7 @@ inline namespace fundamentals_v2
       constexpr propagate_const(_Up&& __u)
 	: _M_t(std::forward<_Up>(__u))
       {}
+
       template <typename _Up, typename
 		enable_if<__and_<is_constructible<_Tp, _Up&&>,
 				 __not_<is_convertible<_Up&&, _Tp>>,
@@ -399,8 +403,10 @@ inline namespace fundamentals_v2
     }
 
   // [propagate_const.algorithms], specialized algorithms
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 3413. propagate_const's swap [...] needs to be constrained and use a trait
   template <typename _Tp>
-    constexpr void
+    constexpr enable_if_t<__is_swappable<_Tp>::value, void>
     swap(propagate_const<_Tp>& __pt, propagate_const<_Tp>& __pt2)
       noexcept(__is_nothrow_swappable<_Tp>::value)
     {
diff --git a/libstdc++-v3/testsuite/experimental/propagate_const/swap/lwg3413.cc b/libstdc++-v3/testsuite/experimental/propagate_const/swap/lwg3413.cc
new file mode 100644
index 00000000000..8dc13cfebdd
--- /dev/null
+++ b/libstdc++-v3/testsuite/experimental/propagate_const/swap/lwg3413.cc
@@ -0,0 +1,41 @@
+// { dg-do compile { target c++14 } }
+
+// LWG 3413
+// propagate_const's swap's noexcept specification needs to be constrained
+// and use a trait
+
+#include <experimental/propagate_const>
+
+using std::experimental::propagate_const;
+
+propagate_const<int*> i;
+static_assert( noexcept(i.swap(i)), "member swap is noexcept" );
+static_assert( noexcept(swap(i, i)), "non-member swap is noexcept" );
+
+struct P
+{
+  int i = 0;
+  int& operator*() const;
+};
+
+void swap(P&, P&) noexcept(false);
+
+propagate_const<P> p;
+static_assert( ! noexcept(p.swap(p)), "member swap is conditionally noexcept" );
+static_assert( ! noexcept(swap(p, p)), "non-member swap is conditionally noexcept" );
+
+// std::is_swappable not available for -std=c++14
+#if __cplusplus > 201402L || !defined(__STRICT_ANSI__)
+struct Q
+{
+  int i = 0;
+  int& operator*() const;
+
+  Q& operator=(Q&&) = delete;
+};
+
+static_assert( ! std::is_swappable<Q>::value, "" );
+
+static_assert( ! std::is_swappable<propagate_const<Q>>::value,
+	       "non-member swap is constrained" );
+#endif