template and derived class definition : error: ‘myClass’ is not a class, namespace, or enumeration

You need to specify the template parameter for it, since myClass is a class template.

template<typename T>
myClass<T>::myClass(void) : std::stack<T>() {
//     ^^^
}

LIVE

^{BTW: : std::stack<T>() seems to be redundant.}