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  1.     #include <type_traits>
  2.  
  3.     template<typename T, typename = void>
  4.     struct is_equality_comparable : std::false_type
  5.     { };
  6.  
  7.     template<typename T>
  8.     struct is_equality_comparable<T,
  9.         typename std::enable_if<
  10.             true,
  11.             decltype((std::declval<T>() == std::declval<T>()), (void)0)
  12.             >::type
  13.         > : std::true_type
  14.     {
  15.     };
  16.  
  17.     struct X { };
  18.     struct Y { };
  19.  
  20.     bool operator == (X const&, X const&) { return true; }
  21.  
  22.     int main()
  23.     {
  24.         static_assert(is_equality_comparable<int>::value, "!");
  25.         static_assert(is_equality_comparable<X>::value, "!");
  26.         static_assert(is_equality_comparable<Y>::value, "!");
  27.     }
  28.  
Compilation error #stdin compilation error #stdout 0s 0KB
comments (?)
stdin
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Standard input is empty
compilation info 
prog.cpp: In function ‘int main()’:
prog.cpp:26:9: error: static assertion failed: !
stdout
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Standard output is empty
https://ideone.com/nuYfaB
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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