Use SFINAE to detect the existence of a templated member function

Question

I learned SFINAE can be used to determine whether a member function exists in a class or not. For example, the following code can be used to check if the method hello is present in a class.

struct has_method_hello {

  using yes = char[1];
  using no  = char[2];

  template <typename U, typename C>
  static constexpr yes& test(decltype(&U::hello));

  template <typename>
  static constexpr no& test(...);

  static constexpr bool value = (sizeof(yes) == sizeof(test<T>(nullptr)));

}; 

struct Foo {
  void hello() {}
}

std::cout << has_method_hello <Foo> :: value << std::endl;  // 1

However, suppose the hello is templated, how can I modify the trick so it can still function properly?

struct Foo {
  template <typename T>
  void hello(T&) {...}
}

Answer 1

From here:

namespace details {
  template<template<class...>class Z, class, class...>
  struct can_apply:std::false_type{};
  template<template<class...>class Z, class...Ts>
  struct can_apply<Z, std::void_t<Z<Ts...>>, Ts...>:
    std::true_type{};
}

template<template<class...>class Z, class...Ts>
using can_apply=details::can_apply<Z, void, Ts...>;

now you want to know if foo.hello(int&) can be called:

We have hello_r that gives you the return type of invoking .hello:

template<class F, class...Ts>
using hello_r = decltype(std::declval<F>().hello( std::declval<Ts>()... ));

which leads to can_hello:

template<class F, class...Ts>
using can_hello = can_apply<hello_r, F, Ts...>;

now

struct Foo {
  template <typename T>
  void hello(T&) {...}
};
int main() {
  std::cout << can_hello<Foo&, int&>::value << '\n';
  std::cout << can_hello<Foo&, char&>::value << '\n';
  std::cout << can_hello<Foo&>::value << '\n';
}

prints 110.

live example.

Answer 2

First of all, showing you a shortened version of your original code:

template <typename T>
struct has_method_hello {

  static constexpr auto test(int) -> decltype(std::declval<T&>().hello(), std::true_type());

  static constexpr std::false_type test(...);

  using result_type = decltype(test(0));
  static const bool value = result_type::value;

};

struct Foo {
  void hello() {}
};

Now making it work for a template parameter, easy, an example:

template <typename T>
struct has_method_hello {

  static constexpr auto test(int) -> decltype(std::declval<T&>().hello(std::declval<int&>()), std::true_type());

  static constexpr std::false_type test(...);

  using result_type = decltype(test(0));
  static const bool value = result_type::value;

};

struct Foo {
  template <typename T>
  void hello(T& v) {}
};

Note that, I have hardcoded int type here. You can make that part of has_method_hello template too.