template<typename T, std::enable_if_t<std::is_same_v<int, T>, T>* = nullptr>
void f() { };

template<typename T, std::enable_if_t<std::is_same_v<double, T>, T>* = nullptr>
void f() { };

int main()
{
    f<int>();
    f<double>();
}

template<typename T, std::enable_if_t<std::is_same_v<int, T>, T>* = nullptr>
struct f { };

template<typename T, std::enable_if_t<std::is_same_v<double, T>, T>* = nullptr>
struct f { };

int main()
{
    f<int>;
    f<double>;
}

main.cpp:36:70: error: template parameter ‘std::enable_if_t<is_same_v<int, T> >* <anonymous>’
 template<typename T, std::enable_if_t<std::is_same_v<int, T>, T>* = nullptr>
                                                                  ^~~~~~~


main.cpp:40:12: error: redeclared here as ‘std::enable_if_t<is_same_v<double, T>, T>* <anonymous>’
 struct f { };
        ^