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#include <iostream>
#include <map>
using namespace std;
 
 
template<typename T>
struct Module
{
	std::map<typename T::Values, int> values;
};
 
struct ConcreteModule : Module<ConcreteModule>
{
	enum class Values
	{
		one,
		two
	};
};
 
int main() {
 
	return 0;
}

prog.cpp: In instantiation of ‘struct Module<ConcreteModule>’:
prog.cpp:12:25:   required from here
prog.cpp:9:36: error: invalid use of incomplete type ‘struct ConcreteModule’
  std::map<typename T::Values, int> values;
                                    ^~~~~~
prog.cpp:12:8: note: forward declaration of ‘struct ConcreteModule’
 struct ConcreteModule : Module<ConcreteModule>
        ^~~~~~~~~~~~~~

Standard output is empty