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#include <cstdint>
 
// Seveval structs which should fail depending if T::f is virtual or not.
template <typename T> struct Dvf : T { void f() final;    };
template <typename T> struct Dvo : T { void f() override; };
template <typename T> struct Dnv : T { void f() = delete; };
 
template <typename U>
class has_virtual_f
{
private:
    template <std::size_t N> struct helper {};
    template <typename T>
    static std::uint8_t check(helper<sizeof(Dvf<T>)>*);
    template<typename T> static std::uint16_t check(...);
public:
    static
    constexpr bool value = sizeof(check<U>(0)) == sizeof(std::uint8_t);
};
 
// Sample struct.
struct V  { virtual void f(); };
struct NV {         void f(); };
struct E  {                   };
struct F { virtual void f() final; }; // Bonus (unspecified expected output)
 
static_assert( has_virtual_f< V>::value, "");
static_assert(!has_virtual_f<NV>::value, "");
static_assert(!has_virtual_f< E>::value, "");
//static_assert(/* ! */ has_virtual_f< F>::value, ""); (unspecified expected output)
 
int main() {
	return 0;
}
 

prog.cpp: In instantiation of ‘struct Dvf<NV>’:
prog.cpp:14:44:   required by substitution of ‘template<class T> static uint8_t has_virtual_f<U>::check(has_virtual_f<U>::helper<sizeof (Dvf<T>)>*) [with T = T; U = NV] [with T = NV]’
prog.cpp:18:40:   required from ‘constexpr const bool has_virtual_f<NV>::value’
prog.cpp:28:35:   required from here
prog.cpp:4:45: error: ‘void Dvf<T>::f() [with T = NV]’ marked final, but is not virtual
 template <typename T> struct Dvf : T { void f() final;    };
                                             ^
prog.cpp:28:1: error: static assertion failed: 
 static_assert(!has_virtual_f<NV>::value, "");
 ^
prog.cpp: In instantiation of ‘struct Dvf<E>’:
prog.cpp:14:44:   required by substitution of ‘template<class T> static uint8_t has_virtual_f<U>::check(has_virtual_f<U>::helper<sizeof (Dvf<T>)>*) [with T = T; U = E] [with T = E]’
prog.cpp:18:40:   required from ‘constexpr const bool has_virtual_f<E>::value’
prog.cpp:29:35:   required from here
prog.cpp:4:45: error: ‘void Dvf<T>::f() [with T = E]’ marked final, but is not virtual
 template <typename T> struct Dvf : T { void f() final;    };
                                             ^
prog.cpp:29:1: error: static assertion failed: 
 static_assert(!has_virtual_f< E>::value, "");
 ^

Standard output is empty