template error message

gistfile1.txt

#include <iostream>
using namespace std;

struct Callable
{
	void operator()(std::string s)	{ cout << "call my struct with " << s << endl; }
};

struct Assignable
{
	Assignable & operator=(const Assignable & other)
	{
		return *this;
	}
};

template<typename T>
auto fcn(T&& caller) -> decltype(declval<T>().operator()(std::declval<std::string>()))
{
	caller("ohh baby");
}

template<typename T>
auto fcn(T&& assignable) -> decltype(declval<T>().operator=(std::declval<Assignable>()))
{
	Assignable other;
	other = assignable;
	cout << "assigning stuff" << endl;
}

template<typename T  = void>
auto fcn(...)
{
	static_assert(!std::is_same<T,T>::value, "Can't call fcn like that. Has to be either callable or assignable");
}

int main() {
	// your code goes here
	
	fcn(Callable());
	fcn(Assignable());
	fcn(string());
	return 0;
}