C++ template parameter deduction based on return type

deduct.cpp

struct Fun {
    // Gadget used for return type based deduction
    // operator T() is probably the only place in C++
    // where the template parameter can be deduced from
    // what the function should return
    template<typename T>
    operator T() {
        return T(sizeof(T));
    }
};
struct Z {
    int a,b,c,d;
    Z(unsigned long x) : a(x) {
    }
};
auto fun() {
    return Fun();
}
int main() {
    Z x = fun(); // 16
    unsigned long y = fun(); // 8 (sizeof(unsigned long))
    int z = fun(); // 4 (sizeof(int))
}