suffering.h

#include <thread>
#include <iostream>
#include <functional>

template<typename T> using Id = T&;

template<typename A, typename B>
struct Phi {
    template<typename R>
    R match(std::function<R(A)>, std::function<R(B)>) {
    }
};

template<typename A, typename B, template<typename, typename> typename Subject, template<typename, typename> typename P> concept Coproduct = requires (Subject<A,B> s) {
    { s.phi() } -> P<A,B>;
    { P<A, B>::match };
};

template<typename A, typename B>
struct Either {
    
    Phi<A,B> phi() {
    }
    
};

template<typename A, typename B>
  requires Coproduct<A, B, Either, Phi>
void witnessEither() {
}

int main() {
    Either<int, double> e;
    witnessEither<int, int>();
}