Attempt to work around C++20 operator rewriting causinginfinite recursion.

Fixes https://github.com/boostorg/utility/issues/65.

appveyor.yml

@@ -15,9 +15,10 @@ branches:
 
 environment:
   matrix:
-    - TOOLSET: msvc-9.0,msvc-10.0,msvc-11.0,msvc-12.0
+    - TOOLSET: msvc-9.0,msvc-10.0,msvc-11.0
+      ADDRMD: 32
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-    - TOOLSET: msvc-14.0
+    - TOOLSET: msvc-12.0,msvc-14.0
       ADDRMD: 32,64
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
     - TOOLSET: msvc-14.1

doc/declval.qbk

@@ -1,6 +1,6 @@
 [/
  / Copyright (c) 2008 Howard Hinnant
- / Copyright (c) 2009-20012 Vicente J. Botet Escriba
+ / Copyright (c) 2009-2012 Vicente J. Botet Escriba
  /
  / Distributed under the Boost Software License, Version 1.0. (See accompanying
  / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

include/boost/operators.hpp

@@ -100,6 +100,11 @@
 #include <boost/config.hpp>
 #include <boost/detail/workaround.hpp>
 #include <boost/core/addressof.hpp>
+#if defined(__cpp_impl_three_way_comparison)
+#include <boost/core/enable_if.hpp>
+#include <boost/type_traits/is_same.hpp>
+#include <boost/type_traits/declval.hpp>
+#endif
 
 #if defined(__sgi) && !defined(__GNUC__)
 #   pragma set woff 1234
@@ -158,9 +163,21 @@ struct less_than_comparable1 : B
 template <class T, class U, class B = operators_detail::empty_base<T> >
 struct equality_comparable2 : B
 {
+#if defined(__cpp_impl_three_way_comparison)
+     template< typename R = decltype(boost::declval< T const& >() == boost::declval< U const& >()) >
+     friend BOOST_OPERATORS_CONSTEXPR
+     typename boost::enable_if_c< !boost::is_same< R, bool >::value, bool >::type
+     operator==(const U& y, const T& x) { return x == y; }
+
+     template< typename R = decltype(boost::declval< T const& >() == boost::declval< U const& >()) >
+     friend BOOST_OPERATORS_CONSTEXPR
+     typename boost::enable_if_c< !boost::is_same< R, bool >::value, bool >::type
+     operator!=(const U& y, const T& x) { return !static_cast<bool>(x == y); }
+#else
      friend BOOST_OPERATORS_CONSTEXPR bool operator==(const U& y, const T& x) { return x == y; }
      friend BOOST_OPERATORS_CONSTEXPR bool operator!=(const U& y, const T& x) { return !static_cast<bool>(x == y); }
-     friend BOOST_OPERATORS_CONSTEXPR bool operator!=(const T& y, const U& x) { return !static_cast<bool>(y == x); }
+#endif
+     friend BOOST_OPERATORS_CONSTEXPR bool operator!=(const T& x, const U& y) { return !static_cast<bool>(x == y); }
 };
 
 template <class T, class B = operators_detail::empty_base<T> >

include/boost/operators_v1.hpp

@@ -144,9 +144,11 @@ struct less_than_comparable1 : B
 template <class T, class U, class B = ::boost::detail::empty_base<T> >
 struct equality_comparable2 : B
 {
+#if !defined(__cpp_impl_three_way_comparison)
      friend bool operator==(const U& y, const T& x) { return x == y; }
      friend bool operator!=(const U& y, const T& x) { return !static_cast<bool>(x == y); }
-     friend bool operator!=(const T& y, const U& x) { return !static_cast<bool>(y == x); }
+#endif
+     friend bool operator!=(const T& x, const U& y) { return !static_cast<bool>(x == y); }
 };
 
 template <class T, class B = ::boost::detail::empty_base<T> >

test/operators_test.cpp

@@ -568,6 +568,25 @@ namespace
         int v;
     };
 
+    // Test type designed specifically to test C++20 operator rewriting support
+    struct my_int :
+        public boost::equality_comparable2< my_int, int >
+    {
+        explicit my_int(int n) : m_n(n) {}
+
+        operator int () const
+        {
+            return m_n;
+        }
+
+        bool operator== (my_int that)
+        {
+            return that.m_n == m_n;
+        }
+
+        int m_n;
+    };
+
 } // unnamed namespace
 
 
@@ -932,5 +951,11 @@ main()
 
     cout << "Performed tests on MyLongInt objects.\n";
 
+    my_int my_n = my_int(10);
+
+    // Test if C++20 operator rewriting causes infinite recursion (https://github.com/boostorg/utility/issues/65)
+    BOOST_TEST(my_n == 10);
+    BOOST_TEST(my_n != 20);
+
     return boost::report_errors();
 }