is_overloaded.hpp

#include <type_traits>
#include <experimental/type_traits>

template <typename T, typename TVoid = void>
struct is_overloaded : std::true_type { };

template <typename T>
struct is_overloaded<T, std::void_t<decltype(&std::decay_t<T>::operator())>>: std::false_type { };

struct s0 
{
  	template <typename T0>
	void operator()(T0){ }
};

struct s1
{  
	void operator()(int){ }
  	void operator()(float){ }
};

struct s2
{  
	void operator()(int){ }  	
};

int main()
{
	auto l0 = [](auto){};
  	auto l1 = [](int){};
  	auto l2 = [](int) -> float {};
  
	static_assert(is_overloaded<decltype(l0)>{}, "");
  	static_assert(!is_overloaded<decltype(l1)>{}, "");
  	static_assert(!is_overloaded<decltype(l2)>{}, "");
  
  	static_assert(is_overloaded<s0>{}, "");
  	static_assert(is_overloaded<s1>{}, "");
  	static_assert(!is_overloaded<s2>{}, "");
}