The type must work with == operator and the result should have standard semantics.
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:

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)
.
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 
(C++20)  specifies that operator == is an equivalence relation (concept) 
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