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#include <iostream>
using namespace std;
 
template <class T>
class Foo
{
	protected:
	static T* t;
	friend T;
};
template <class T>
T* Foo<T>::t = nullptr;
 
template <class Self, class T> //T = Type derived from Foo
struct Bar
{
	void test()
	{
		T::t = static_cast<Self*>(this);
	}
};
 
struct FooDerived;
 
struct BarDerived : public Bar<BarDerived, FooDerived>
{
 
};
 
struct FooDerived : public Foo<BarDerived>
{
 
};
 
int main() {
 
	auto a = BarDerived{};
	a.test();
 
	return 0;
}

prog.cpp: In instantiation of ‘void Bar<Self, T>::test() [with Self = BarDerived; T = FooDerived]’:
prog.cpp:38:9:   required from here
prog.cpp:12:4: error: ‘BarDerived* Foo<BarDerived>::t’ is protected
 T* Foo<T>::t = nullptr;
    ^
prog.cpp:19:8: error: within this context
   T::t = static_cast<Self*>(this);
        ^

Standard output is empty