prog.cc: In function 'int main()':
prog.cc:14:23: error: passing 'const Integer' as 'this' argument discards qualifiers [-fpermissive]
static_assert(x.get() == 3, "x value must be 3");
^
prog.cc:7:17: note: in call to 'constexpr int Integer::get()'
constexpr int get()
^~~