--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libstdc++-v3/testsuite/23_containers/set/operators/cmp_c++20.cc	Mon May 25 07:50:57 2020 +0900
@@ -0,0 +1,107 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <set>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::set<int> c1{ 1, 2, 3 }, c2{ 1, 2, 3, 4 }, c3{ 1, 2, 4 };
+  VERIFY( c1 == c1 );
+  VERIFY( std::is_eq(c1 <=> c1) );
+  VERIFY( c1 < c2 );
+  VERIFY( std::is_lt(c1 <=> c2) );
+  VERIFY( c1 < c3 );
+  VERIFY( std::is_lt(c1 <=> c3) );
+  VERIFY( c2 < c3 );
+  VERIFY( std::is_lt(c2 <=> c3) );
+
+  static_assert( std::totally_ordered<std::set<int>> );
+
+  static_assert( std::three_way_comparable<std::set<int>,
+					   std::strong_ordering> );
+  static_assert( ! std::three_way_comparable<std::set<float>,
+					     std::strong_ordering> );
+  static_assert( ! std::three_way_comparable<std::set<float>,
+					     std::weak_ordering> );
+  static_assert( std::three_way_comparable<std::set<float>,
+					   std::partial_ordering> );
+
+  struct E
+  {
+    bool operator==(E) { return true; }
+  };
+  static_assert( ! std::totally_ordered<std::set<E>> );
+  static_assert( ! std::three_way_comparable<E> );
+  static_assert( ! std::three_way_comparable<std::set<E>> );
+}
+
+void
+test02()
+{
+  struct W
+  {
+    int value = 0;
+
+    bool operator==(W rhs) const noexcept
+    { return (value | 1) == (rhs.value | 1); }
+
+    std::weak_ordering
+    operator<=>(W rhs) const noexcept
+    { return (value | 1) <=> (rhs.value | 1); }
+  };
+
+  static_assert( std::totally_ordered<std::set<W>> );
+
+  std::set<W> c1{ {1}, {2}, {3} }, c2{ {0}, {3}, {3} };
+  static_assert( std::same_as<decltype(c1 <=> c1), std::weak_ordering> );
+  VERIFY( c1 == c2 );
+  VERIFY( std::is_eq(c1 <=> c2) );
+}
+
+void
+test04()
+{
+  struct L
+  {
+    int value = 0;
+
+    bool operator<(L rhs) const noexcept { return value < rhs.value; }
+  };
+
+  static_assert( std::totally_ordered<std::set<L>> );
+
+  std::set<L> c{ {1}, {2}, {3} }, d{ {1}, {2}, {3}, {4} };
+  static_assert( std::same_as<decltype(c <=> c), std::weak_ordering> );
+  VERIFY( std::is_lt(c <=> d) );
+}
+
+// Associative container iterators are not random access
+static_assert( ! std::totally_ordered<std::set<int>::iterator> );
+static_assert( ! std::three_way_comparable<std::set<int>::iterator> );
+
+int
+main()
+{
+  test01();
+  test02();
+  test04();
+}