Fix GCC 13 warning about ambiguity of operator==() in C++20

In C++20, a==b can also be implicitly rewritten as b==a, assuming
commutativity. But if the operator==(const &) is not declared as const, and is
thus asymmetric, this results in an ambiguity because neither of a==b or b==a
is a better match according to the overload resolution rules.