Until C++14 constexpr on a non-static member function did actually imply const, since mutation was (logically enough) not permitted in constexpr functions. C++1

Until C++14 constexpr on a non-static member function did actually imply const, since mutation was (logically enough) not permitted in constexpr functions. C++14 relaxed this restriction, however, by allowing constexpr functions to mutate objects which only exist within the lifetime of the constexpr expression, so constexpr no longer implies const.