@tuxcrafting @caseyp

constexpr applies to the function and means "compiler may execute this at compile time if it feels like it or has to" and it applies to the function. This error seems to be about the "this" argument, the special hidden argument every member function has, that you can qualify with const/volatile/ref(&)/rvalue_ref(&&) after the parameter list, usually before noexcept (though not sure if that last part is a hard rule). Default is reference (not const, not volatile, not rvalue).

Examples:

// const ref
bool operator==(uter) const;

// rvalue ref
bool operator==(uter) &&;

// explicit ref to overload rvalue ref
bool operator==(uter) &;