ceph/src/boost/boost/poly_collection/detail/is_equality_comparable.hpp

   1 /* Copyright 2017 Joaquin M Lopez Munoz.
   2  * Distributed under the Boost Software License, Version 1.0.
   3  * (See accompanying file LICENSE_1_0.txt or copy at
   4  * http://www.boost.org/LICENSE_1_0.txt)
   5  *
   6  * See http://www.boost.org/libs/poly_collection for library home page.
   7  */
   8
   9 #ifndef BOOST_POLY_COLLECTION_DETAIL_IS_EQUALITY_COMPARABLE_HPP
  10 #define BOOST_POLY_COLLECTION_DETAIL_IS_EQUALITY_COMPARABLE_HPP
  11
  12 #if defined(_MSC_VER)
  13 #pragma once
  14 #endif
  15
  16 #include <boost/config.hpp>
  17 #include <type_traits>
  18
  19 #if !defined(BOOST_NO_SFINAE_EXPR)
  20 #include <utility>
  21 #else
  22 #include <boost/poly_collection/detail/is_likely_stateless_lambda.hpp>
  23 #include <boost/type_traits/has_equal_to.hpp>
  24 #endif
  25
  26 namespace boost{
  27
  28 namespace poly_collection{
  29
  30 namespace detail{
  31
  32 #if !defined(BOOST_NO_SFINAE_EXPR)
  33
  34 /* trivial, expression SFINAE-based implementation */
  35
  36 template<typename T,typename=void>
  37 struct is_equality_comparable:std::false_type{};
  38
  39 template<typename T>
  40 struct is_equality_comparable<
  41   T,
  42   typename std::enable_if<
  43     std::is_convertible<
  44       decltype(std::declval<T>()==std::declval<T>()),bool
  45     >::value
  46   >::type
  47 >:std::true_type{};
  48
  49 #else
  50 /* boost::has_equal_to does a decent job without using expression SFINAE,
  51  * but it produces a compile error when the type T being checked is
  52  * convertible to an equality-comparable type Q. Exotic as it may seem,
  53  * this is exactly the situation with the very important case of stateless
  54  * lambda expressions, which are convertible to an equality-comparable
  55  * function pointer with the same signature. We take explicit care of
  56  * stateless lambdas then.
  57  */
  58
  59 template<typename T,typename=void>
  60 struct is_equality_comparable:std::integral_constant<
  61   bool,
  62   has_equal_to<T,T,bool>::value
  63 >{};
  64
  65 template<typename T>
  66 struct is_equality_comparable<
  67   T,
  68   typename std::enable_if<is_likely_stateless_lambda<T>::value>::type
  69 >:
  70 #if !defined(BOOST_MSVC)
  71   std::true_type{};
  72 #else
  73   /* To complicate things further, in VS stateless lambdas are convertible not
  74    * only to regular function pointers, but also to other call conventions
  75    * such as __stdcall, __fastcall, etc., which makes equality comparison
  76    * ambiguous.
  77    */
  78
  79   std::false_type{};
  80 #endif
  81 #endif
  82
  83 } /* namespace poly_collection::detail */
  84
  85 } /* namespace poly_collection */
  86
  87 } /* namespace boost */
  88
  89 #endif