Error(s):

source_file.cpp:19:9: error: static_assert failed "Not all args as Double"
        static_assert(all_same<double, Args...>{}, "Not all args as Double");
        ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~
source_file.cpp:29:6: note: in instantiation of function template specialization 'myclass<3>::mymethod<char, double, double>' requested here
   x.mymethod('q', 1., 7.); // This doesn't work
     ^
1 error generated.