main.cpp

#include <iostream>
#include <type_traits>
#include <string>
#include <typeinfo>

using namespace std;

struct Obj {};

struct ObjWithFoo {
	using foo = char;
};

template <typename T, typename>
struct helper_foo : false_type {};

template <typename T>
struct helper_foo<T, decltype(((void)declval<typename T::foo>(), 1))> : true_type {};

template <typename T>
struct has_foo {
	static const bool value = helper_foo<T, int>::value;
};

template <typename T, typename = T>
struct helper_fallback : true_type{};

template <typename T>
struct helper_fallback<T, enable_if_t<is_integral<T>::value, T>> : false_type{};

template <typename T>
struct helper_fallback<T, enable_if_t<is_floating_point<T>::value, T>> : false_type {};

template <typename T>
struct is_fallback {
	static constexpr bool value = helper_fallback<T>::value;
};

template <typename T>
enable_if_t<is_integral<T>::value, void> test(T t) {
	cout << "is_integeral<T>::value == true" << endl;
}

template <typename T>
enable_if_t<is_floating_point<T>::value, void> test(T t) {
	cout << "is_floating_point<T>::value == true" << endl;
}

template <typename T>
enable_if_t<is_fallback<T>::value && has_foo<T>::value, void> test(T t) {
	cout << "fallback -- T::foo exists" << endl;
}

template <typename T>
enable_if_t<is_fallback<T>::value && !has_foo<T>::value, void> test(T t) {
	cout << "fallback -- T::foo does not exist" << endl;
}

template <typename T>
void multi_test(T t) {
	test(t);
}

template <typename T, typename... Args>
void multi_test(T t, Args... args) {
	multi_test(t);
	multi_test(args...);
}

int main() {
	int i = 10;
	double d = 20.3;
	Obj o;
	ObjWithFoo o2;

	multi_test(i, d, o, o2);

	return 0;
}