U does not name a type - template compilation error

Here is my simple program. I tried forcing the static_cast so that the compiler knows that U is nothing but int. However, still, the template cannot instantiate the int version of my function Add.

In the future, I want to extend this template so that U and V are of different types and my program still works.

However, currently, even the same type is not working. What am I missing here?

What is the reason for the compilation error?

#include <iostream>
using namespace std;

template <typename U>
class Addable
{
    U x;
public:
    Addable(U val) : x{ val } { cout << "Constructor called" << endl; }
    template <typename T>
    U add(T y);
};

U Addable::add(T y)
{
    x += y;
    return x;
}

int main()
{
    Addable<int> A{ 10 };
    int x = A.add(static_cast<int>(10));
    cout << x;
    return 0;
}