#include <unordered_map>
#include <unordered_set>

struct elem { 
    elem(unsigned val) : value(val) {}
    unsigned value = 0;
};
struct hash {
    using is_transparent = void;
    unsigned operator()(elem e) const { return e.value; }
};
struct equal {
    using is_transparent = void;
    unsigned operator()(elem e1, elem e2) const { return e1.value == e2.value; }
};

int main() {
    std::unordered_map<elem, int, hash, equal> map;
    (void)map.find(1u);   // OK
    (void)map.find({1u}); // MSVC: compile error, GCC: OK
    std::unordered_set<elem, hash, equal> set;
    (void)set.find(1u);   // OK
    (void)set.find({1u}); // MSVC: compile error, GCC: OK
}

#include <unordered_set>

int main()
{
    std::unordered_set<int> m{1, 2, 3};
    auto it = m.find<void()>(1);
    return it == m.end();
}