test.cpp

#include <type_traits>

using std::is_callable_v;

template <typename Fn, typename A> constexpr auto bind(Fn fn, A &&a) noexcept {
  return [fn, a](auto &&... args) { return fn(a, args...); };
}

int main() {
  constexpr auto b0 = bind([]() {}, 4);
  constexpr auto b2 = bind([](int, int) {}, 4);

  // OK.
  static_assert(is_callable_v<decltype(b2)(int)>);

  // FAILING to compile.
  static_assert(!is_callable_v<decltype(b0)()>);
  static_assert(!is_callable_v<decltype(b2)()>);
  static_assert(!is_callable_v<decltype(b2)(int, int)>);
}