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

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

CMakeLists.txt

@@ -3,9 +3,12 @@
 # Distributed under the Boost Software License, Version 1.0.
 # See accompanying file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt
 
-cmake_minimum_required(VERSION 3.5...3.20)
+# Partial (add_subdirectory only) and experimental CMake support
+# Subject to change; please do not rely on the contents of this file yet.
 
-project(boost_utility VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)
+cmake_minimum_required(VERSION 3.5)
+
+project(BoostUtility LANGUAGES CXX)
 
 add_library(boost_utility INTERFACE)
 add_library(Boost::utility ALIAS boost_utility)
@@ -15,6 +18,7 @@ target_include_directories(boost_utility INTERFACE include)
 target_link_libraries(boost_utility
     INTERFACE
         Boost::config
+        Boost::container_hash
         Boost::core
         Boost::io
         Boost::preprocessor

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
@@ -112,8 +117,6 @@
 // Define BOOST_OPERATORS_CONSTEXPR to be like BOOST_CONSTEXPR but empty under MSVC < v19.22
 #if BOOST_WORKAROUND(BOOST_MSVC, < 1922)
 #define BOOST_OPERATORS_CONSTEXPR
-#elif defined __sun
-#define BOOST_OPERATORS_CONSTEXPR
 #else
 #define BOOST_OPERATORS_CONSTEXPR BOOST_CONSTEXPR
 #endif
@@ -160,9 +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> >

include/boost/utility/detail/result_of_variadic.hpp

@@ -1,190 +0,0 @@
-// Boost result_of library
-
-//  Copyright Douglas Gregor 2004. Use, modification and
-//  distribution is subject to 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)
-
-//  Copyright Daniel Walker, Eric Niebler, Michel Morin 2008-2012.
-//  Use, modification and distribution is subject to 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)
-
-// For more information, see http://www.boost.org/libs/utility
-
-#ifndef BOOST_RESULT_OF_HPP
-# error Boost result_of - do not include this file!
-#endif
-
-template<typename F, typename... Args>
-struct tr1_result_of<F(Args...)>
-    : conditional<
-        is_pointer<F>::value || is_member_function_pointer<F>::value
-        , boost::detail::tr1_result_of_impl<
-            typename remove_cv<F>::type,
-            typename remove_cv<F>::type(Args...),
-            (boost::detail::result_of_has_result_type<F>::value)>
-        , boost::detail::tr1_result_of_impl<
-            F,
-            F(Args...),
-            (boost::detail::result_of_has_result_type<F>::value)> >::type { };
-
-#ifdef BOOST_RESULT_OF_USE_DECLTYPE
-template<typename F, typename... Args>
-struct result_of<F(Args...)>
-    : detail::cpp0x_result_of<F(Args...)> { };
-#endif // BOOST_RESULT_OF_USE_DECLTYPE
-
-#ifdef BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK
-template<typename F, typename... Args>
-struct result_of<F(Args...)>
-    : conditional<detail::result_of_has_result_type<F>::value || detail::result_of_has_result<F>::value,
-               tr1_result_of<F(Args...)>,
-               detail::cpp0x_result_of<F(Args...)> >::type { };
-#endif // BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK
-
-#if defined(BOOST_RESULT_OF_USE_DECLTYPE) || defined(BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK)
-
-namespace detail {
-
-template<typename F, typename... Args>
-struct cpp0x_result_of<F(Args...)>
-    : conditional<
-          is_member_function_pointer<F>::value
-        , detail::tr1_result_of_impl<
-            typename remove_cv<F>::type,
-            typename remove_cv<F>::type(Args...), false
-          >
-        , detail::cpp0x_result_of_impl<
-              F(Args...)
-          >
-      >::type
-{};
-
-#ifdef BOOST_NO_SFINAE_EXPR
-
-template<typename F>
-struct result_of_callable_fun_2;
-
-template<typename R, typename... Args>
-struct result_of_callable_fun_2<R(Args...)> {
-    R operator()(Args...) const;
-    typedef result_of_private_type const &(*pfn_t)(...);
-    operator pfn_t() const volatile;
-};
-
-template<typename F>
-struct result_of_callable_fun
-  : result_of_callable_fun_2<F>
-{};
-
-template<typename F>
-struct result_of_callable_fun<F *>
-  : result_of_callable_fun_2<F>
-{};
-
-template<typename F>
-struct result_of_select_call_wrapper_type
-  : conditional<
-        is_class<typename remove_reference<F>::type>::value,
-        result_of_wrap_callable_class<F>,
-        type_identity<result_of_callable_fun<typename remove_cv<typename remove_reference<F>::type>::type> >
-    >::type
-{};
-
-template<typename F, typename... Args>
-struct result_of_is_callable {
-    typedef typename result_of_select_call_wrapper_type<F>::type wrapper_t;
-    static const bool value = (
-        sizeof(result_of_no_type) == sizeof(detail::result_of_is_private_type(
-            (boost::declval<wrapper_t>()(boost::declval<Args>()...), result_of_weird_type())
-        ))
-    );
-    typedef integral_constant<bool, value> type;
-};
-
-template<typename F, typename... Args>
-struct cpp0x_result_of_impl<F(Args...), true>
-    : lazy_enable_if<
-          result_of_is_callable<F, Args...>
-        , cpp0x_result_of_impl<F(Args...), false>
-      >
-{};
-
-template<typename F, typename... Args>
-struct cpp0x_result_of_impl<F(Args...), false>
-{
-  typedef decltype(
-    boost::declval<F>()(
-      boost::declval<Args>()...
-    )
-  ) type;
-};
-
-#else // BOOST_NO_SFINAE_EXPR
-
-template<typename F, typename... Args>
-struct cpp0x_result_of_impl<F(Args...),
-                            typename result_of_always_void<decltype(
-                                boost::declval<F>()(
-                                    boost::declval<Args>()...
-                                )
-                            )>::type> {
-  typedef decltype(
-    boost::declval<F>()(
-      boost::declval<Args>()...
-    )
-  ) type;
-};
-
-#endif // BOOST_NO_SFINAE_EXPR
-
-} // namespace detail
-
-#else // defined(BOOST_RESULT_OF_USE_DECLTYPE) || defined(BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK)
-
-template<typename F, typename... Args>
-struct result_of<F(Args...)>
-    : tr1_result_of<F(Args...)> { };
-
-#endif // defined(BOOST_RESULT_OF_USE_DECLTYPE)
-
-namespace detail {
-
-template<typename R, typename FArgs, typename... Args>
-struct tr1_result_of_impl<R (*)(Args...), FArgs, false>
-{
-  typedef R type;
-};
-
-template<typename R, typename FArgs, typename... Args>
-struct tr1_result_of_impl<R (&)(Args...), FArgs, false>
-{
-  typedef R type;
-};
-
-template<typename R, typename FArgs, typename C, typename... Args>
-struct tr1_result_of_impl<R (C::*)(Args...), FArgs, false>
-{
-  typedef R type;
-};
-
-template<typename R, typename FArgs, typename C, typename... Args>
-struct tr1_result_of_impl<R (C::*)(Args...) const, FArgs, false>
-{
-  typedef R type;
-};
-
-template<typename R, typename FArgs, typename C, typename... Args>
-struct tr1_result_of_impl<R (C::*)(Args...) volatile, FArgs, false>
-{
-  typedef R type;
-};
-
-template<typename R, typename FArgs, typename C, typename... Args>
-struct tr1_result_of_impl<R (C::*)(Args...) const volatile, FArgs, false>
-{
-  typedef R type;
-};
-
-}

include/boost/utility/result_of.hpp

@@ -10,6 +10,13 @@
 #define BOOST_RESULT_OF_HPP
 
 #include <boost/config.hpp>
+#include <boost/preprocessor/cat.hpp>
+#include <boost/preprocessor/iteration/iterate.hpp>
+#include <boost/preprocessor/repetition/enum_params.hpp>
+#include <boost/preprocessor/repetition/enum_trailing_params.hpp>
+#include <boost/preprocessor/repetition/enum_binary_params.hpp>
+#include <boost/preprocessor/repetition/enum_shifted_params.hpp>
+#include <boost/preprocessor/facilities/intercept.hpp>
 #include <boost/detail/workaround.hpp>
 #include <boost/type_traits/is_class.hpp>
 #include <boost/type_traits/is_pointer.hpp>
@@ -22,20 +29,6 @@
 #include <boost/type_traits/integral_constant.hpp>
 #include <boost/core/enable_if.hpp>
 
-#ifdef BOOST_NO_CXX11_VARIADIC_TEMPLATES
-#  undef BOOST_RESULT_OF_NO_VARIADIC_TEMPLATES
-#  define BOOST_RESULT_OF_NO_VARIADIC_TEMPLATES
-#endif
-#ifdef BOOST_RESULT_OF_NO_VARIADIC_TEMPLATES
-#  include <boost/preprocessor/cat.hpp>
-#  include <boost/preprocessor/iteration/iterate.hpp>
-#  include <boost/preprocessor/repetition/enum_params.hpp>
-#  include <boost/preprocessor/repetition/enum_trailing_params.hpp>
-#  include <boost/preprocessor/repetition/enum_binary_params.hpp>
-#  include <boost/preprocessor/repetition/enum_shifted_params.hpp>
-#  include <boost/preprocessor/facilities/intercept.hpp>
-#endif
-
 #ifndef BOOST_RESULT_OF_NUM_ARGS
 #  define BOOST_RESULT_OF_NUM_ARGS 16
 #endif
@@ -224,12 +217,8 @@ struct tr1_result_of_impl<F, FArgs, false>
 
 } // end namespace detail
 
-#ifndef BOOST_RESULT_OF_NO_VARIADIC_TEMPLATES
-#  include <boost/utility/detail/result_of_variadic.hpp>
-#else
-#  define BOOST_PP_ITERATION_PARAMS_1 (3,(0,BOOST_RESULT_OF_NUM_ARGS,<boost/utility/detail/result_of_iterate.hpp>))
-#  include BOOST_PP_ITERATE()
-#endif
+#define BOOST_PP_ITERATION_PARAMS_1 (3,(0,BOOST_RESULT_OF_NUM_ARGS,<boost/utility/detail/result_of_iterate.hpp>))
+#include BOOST_PP_ITERATE()
 
 #if 0
 // inform dependency trackers, as they can't see through macro includes

include/boost/utility/string_view.hpp

@@ -23,6 +23,7 @@
 #include <boost/io/ostream_put.hpp>
 #include <boost/utility/string_view_fwd.hpp>
 #include <boost/throw_exception.hpp>
+#include <boost/container_hash/hash_fwd.hpp>
 
 #include <cstddef>
 #include <stdexcept>
@@ -651,9 +652,6 @@ namespace boost {
         }
 #endif
 
-    // Forward declaration of Boost.ContainerHash function
-    template <class It> std::size_t hash_range(It, It);
-
     template <class charT, class traits>
     std::size_t hash_value(basic_string_view<charT, traits> s) {
         return boost::hash_range(s.begin(), s.end());

meta/libraries.json

@@ -13,8 +13,7 @@
             "Memory",
             "Miscellaneous",
             "Patterns"
-        ],
-        "cxxstd": "03"
+        ]
     },
     {
         "key": "utility/call_traits",
@@ -26,8 +25,7 @@
         "documentation": "call_traits.htm",
         "category": [
             "Generic"
-        ],
-        "cxxstd": "03"
+        ]
     },
     {
         "key": "utility/compressed_pair",
@@ -40,8 +38,7 @@
         "category": [
             "Data",
             "Patterns"
-        ],
-        "cxxstd": "03"
+        ]
     },
     {
         "key": "utility/identity_type",
@@ -56,8 +53,7 @@
         ],
         "maintainers": [
             "Lorenzo Caminiti <lorcaminiti -at- gmail.com>"
-        ],
-        "cxxstd": "03"
+        ]
     },
     {
         "key": "utility/in_place_factories",
@@ -69,8 +65,7 @@
         "documentation": "in_place_factories.html",
         "category": [
             "Generic"
-        ],
-        "cxxstd": "03"
+        ]
     },
     {
         "key": "utility/operators",
@@ -88,8 +83,7 @@
         ],
         "maintainers": [
             "Daniel Frey <d.frey -at- gmx.de>"
-        ],
-        "cxxstd": "03"
+        ]
     },
     {
         "key": "utility/result_of",
@@ -102,8 +96,7 @@
         "authors": "",
         "maintainers": [
             "Daniel Walker <daniel.j.walker -at- gmail.com>"
-        ],
-        "cxxstd": "03"
+        ]
     },
     {
         "key": "utility/string_ref",
@@ -116,8 +109,7 @@
         "authors": "Marshall Clow",
         "maintainers": [
             "Marshall Clow <marshall -at- idio.com>"
-        ],
-        "cxxstd": "03"
+        ]
     },
     {
         "key": "utility/value_initialized",
@@ -129,7 +121,6 @@
         "documentation": "value_init.htm",
         "category": [
             "Miscellaneous"
-        ],
-        "cxxstd": "03"
+        ]
     }
 ]

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();
 }