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    #include <iostream>
    using namespace std;
 
    template <typename T>
    class Singleton
    {
    public:
    	static T& Get() noexcept
    	{
    		static T instance;
    		return instance;
    	};
 
    protected:
    	Singleton() = default;
    };
 
    class Foo : public Singleton<Foo>
    {
    public:
    	int t = 0;
    private:
    	Foo() = default;
    };
 
    int main() {
 
    	//Foo foo;
    	Foo::Get().t += 10;
 
    	return 0;
    }

prog.cpp: In instantiation of ‘static T& Singleton<T>::Get() [with T = Foo]’:
prog.cpp:29:11:   required from here
prog.cpp:10:16: error: ‘constexpr Foo::Foo()’ is private within this context
       static T instance;
                ^~~~~~~~
prog.cpp:23:6: note: declared private here
      Foo() = default;
      ^~~

Standard output is empty