#include <iostream>
#include <type_traits>
 
template<class D>
class widget
{
public:
	void foo() { self().bar(); }
 
private:
	D const& self() const
	{
		static_assert(std::is_base_of<widget, D>(), "D must derive");
		return (D const&)*this;
	}
 
	D& self()
	{
		auto const& that = *this;
		return const_cast<D&>(that.self());
	}
};
 
struct gadget : private widget<gadget>
{
	using widget::foo;
	void bar() { std::cout << "gadget\n"; }
};
 
int main()
{
	gadget g;
	g.foo();
}
 

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