C++ named requirements: EqualityComparable
From cppreference.com
The type must work with == operator and the result should have standard semantics.
Contents 
[edit] Requirements
The type T
satisfies EqualityComparable if
Given
 a, b and c, expressions of type
T
or(since C++11)const T
.
The following expressions must be valid and have their specified effects:
Expression  Return type  Requirements 

a == b  implicitly convertible to bool  Establishes an equivalence relation, that is, it satisfies the following properties:

[edit] Notes
To satisfy this requirement, types that do not have builtin comparison operators have to provide a userdefined operator==.
For the types that are both EqualityComparable and LessThanComparable, the C++ standard library makes a distinction between equality, which is the value of the expression a == b and equivalence, which is the value of the expression !(a < b) && !(b < a).
[edit] Defect reports
The following behaviorchanging defect reports were applied retroactively to previously published C++ standards.
DR  Applied to  Behavior as published  Correct behavior 

LWG 283  C++98  even if T is EqualityComparable, the requirements did not apply to const T objects

they apply to const T instead of T

[edit] See also
specifies that operator == is an equivalence relation (concept) 