#if 0
set -xe
g++     -std=c++11 -fsyntax-only fn_signature_checker.cc || echo "EXPECTED FAILURE"
clang++ -std=c++11 -fsyntax-only fn_signature_checker.cc || echo "EXPECTED FAILURE"
g++     -std=c++20 -fsyntax-only fn_signature_checker.cc || echo "EXPECTED FAILURE"
clang++ -std=c++20 -fsyntax-only fn_signature_checker.cc || echo "EXPECTED FAILURE"
exit 0
#endif

#include <type_traits>

// Function signature checker for generic functions.
//
// This demonstrates a way to do some simple function signature checking on
// template arguments w/o implementing the whole concept emulation with
// std::void_t prior to cpp20. This may not be the best solution, but it gives a
// small tool at hand which can easily be employed into older cpp code bases.
//
// Starting with cpp20, one can fully switch to concepts for this task.

// -- PRE CPP20 ----------------------------------------------------------------

// (1) Primary template to extract the function signature type.
template <typename Fn>
struct fn;

// (2) Partial specialization for function types.
template <typename Ret, typename... Args>
struct fn<Ret(Args...)> {
  using type = Ret(Args...);
};

// (3) Partial specialization for function pointer types.
//
// This specialization is disabled on purpose as it allows for writing ambiguous
// checks for either non-static or static member functions.
//
// template <typename Ret, typename... Args>
// struct fn<Ret (*)(Args...)> {
//   using type = Ret(Args...);
// };

// (4) Partial specialization for member function pointer types.
template <typename T, typename Ret, typename... Args>
struct fn<Ret (T::*)(Args...)> {
  using type = Ret(Args...);
};

template <typename Fn>
using fn_t = typename fn<Fn>::type;

template <typename T>
void do_work() {
  // Check for non-static member function.
  //
  // If the partial specialization (3) is enabled, this could also match a
  // static member function T::get.
  static_assert(std::is_same<int(int), fn_t<decltype(&T::get)>>::value,
                "T has no member function 'int get(int)'");
  // Check for non-static member function.
  static_assert(std::is_same<void(int), fn_t<decltype(&T::set)>>::value,
                "T has no member function 'void set(int)'");
  // Check for static member function.
  static_assert(std::is_same<int(), fn_t<decltype(T::cnt)>>::value,
                "T has no static function 'static int cnt()'");
}

struct good {
  int get(int);
  void set(int);
  static int cnt();
};

struct bad {
  int get(int);
  int set();
  static int cnt();
};

void check() {
  do_work<good>();

  // TODO: Enable to see a failing check due to wrong function signature.
  // do_work<bad>();
}

// -- SINCE CPP20 --------------------------------------------------------------

#if __cplusplus >= 202002L
#include <concepts>

template <typename T>
concept MyType = requires(T t) {
  { t.get(int{}) } -> std::same_as<int>;
  { t.set(int{}) } -> std::same_as<void>;
  { T::cnt() } -> std::same_as<int>;
};

template <MyType T>
void do_work2() {
}

void check2() {
  do_work2<good>();

  // TODO: Enable to see failing to satisfy the concept MyType.
  // do_work2<bad>();
}

#endif