11 Classes [class]

11.11 Comparisons [class.compare]

11.11.2 Equality operator [class.eq]

A defaulted equality operator function ([over.binary]) shall have a declared return type bool.
A defaulted == operator function for a class C is defined as deleted unless, for each in the expanded list of subobjects for an object x of type C, is usable ([class.compare.default]).
The return value V of a defaulted == operator function with parameters x and y is determined by comparing corresponding elements and in the expanded lists of subobjects for x and y (in increasing index order) until the first index i where yields a result value which, when contextually converted to bool, yields false.
If no such index exists, V is true.
Otherwise, V is false.
struct D {
  int i;
  friend bool operator==(const D& x, const D& y) = default;
                                                // OK, returns x.i == y.i
— end example