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#include <iostream>
#include <type_traits>
using namespace std;
 
template <typename T>
struct dummy {};
 
template <typename T, typename U>
struct fail : dummy<T>, dummy<U> {};
 
template <typename T>
struct ok : dummy<T> {};
 
template <typename T>
true_type test(fail<T, T> a = fail<T, T>());
 
//template <typename T>
//true_type test(ok<T> a = ok<T>());
 
false_type test(...);
 
int main() {
	cout << decltype(test<int>())::value << endl;
}

prog.cpp: In instantiation of ‘struct fail<int, int>’:
prog.cpp:23:29:   required from here
prog.cpp:9:8: error: duplicate base type ‘dummy<int>’ invalid
 struct fail : dummy<T>, dummy<U> {};
        ^
prog.cpp: In function ‘std::true_type test(fail<T, T>) [with T = int; std::true_type = std::integral_constant<bool, true>]’:
prog.cpp:23:29: note:   when instantiating default argument for call to std::true_type test(fail<T, T>) [with T = int; std::true_type = std::integral_constant<bool, true>]
  cout << decltype(test<int>())::value << endl;
                             ^

Standard output is empty