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#include <iostream>
#include <type_traits>
 
using namespace std;
 
class Incomplete;
class Complete {};
 
template <typename IncompleteType>
struct DetermineCompleteHelper : public IncompleteType {};
 
template <typename IncompleteType, typename = std::enable_if_t<true>>
struct DetermineComplete {
    static constexpr const bool value = false;
};
 
template <typename IncompleteType>
struct DetermineComplete<IncompleteType, std::enable_if_t<std::is_same<
        decltype(DetermineCompleteHelper<IncompleteType>{}),
        decltype(DetermineCompleteHelper<IncompleteType>{})>::value>> {
    static constexpr const bool value = true;
};
 
int main() {
    cout << DetermineComplete<Complete>::value << endl;
    cout << DetermineComplete<Incomplete>::value << endl;
    return 0;
}
 

prog.cpp: In instantiation of ‘struct DetermineCompleteHelper<Incomplete>’:
prog.cpp:26:42:   required from here
prog.cpp:10:8: error: invalid use of incomplete type ‘class Incomplete’
 struct DetermineCompleteHelper : public IncompleteType {};
        ^~~~~~~~~~~~~~~~~~~~~~~
prog.cpp:6:7: note: forward declaration of ‘class Incomplete’
 class Incomplete;
       ^~~~~~~~~~

Standard output is empty