#include <iostream>
#include <type_traits>
 
using namespace std;
 
struct FooNo {};
struct FooYes { int x; };
 
struct FooDerivedYes : public FooYes {};
struct FooDerivedNo : public FooNo {};
struct FooDerivedTwiceYes final : public FooYes { int x; };
 
template <typename T, typename = void>
struct HasX : false_type { };
 
template <typename T>
struct HasX <T, decltype((void) T::x)> : true_type { };
 
int main() {
    cout << HasX<FooYes>::value << endl;
    cout << HasX<FooNo>::value << endl;
    cout << HasX<FooDerivedYes>::value << endl;
    cout << HasX<FooDerivedNo>::value << endl;
    cout << HasX<FooDerivedTwiceYes>::value << endl;
}

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