Where to put non-member function definition of a PRIVATE nested (inner)class?

Following is a simple example of my actual problem. I have a non-member function defined using friend keyword in the InnerClass. The definition of this function should be outside of InnerClass.

On the other hand, the class definition is under private of MyClass as I want to use the InnerClass specifically for MyClass.

In this case, where I can actually put the definition of friend function?

#include <iostream>

class MyClass
{
private:
    class InnerClass
    {
        int m_int;
    public:
        InnerClass(int i) :m_int(i) {}

        // non-member (friend) function, for which I need a definition outside the class!
        friend std::ostream& operator<<(std::ostream& output, const InnerClass &obj);
    };
    // ERROR: too many parameter for this operator
    std::ostream& operator<<(std::ostream& output, const InnerClass &obj)
    {
        return output << obj.m_int << " ";
    }

private:
    InnerClass m_innerClass;

public:
    explicit MyClass(int i) : m_innerClass{ i } {}
    friend std::ostream& operator<<(std::ostream& output, const MyClass &obj);
};

std::ostream& operator<<(std::ostream& output, const MyClass &obj)
{
    output << obj.m_innerClass << std::endl;
}


int main()
{
    MyClass classObj{ 2 };
    std::cout << classObj;
    return 0;
}

The error msg:

  \main.cpp(27): error C2804: binary 'operator <<' has too many parameters
  \main.cpp(27): error C2333: 'MyClass::operator <<': error in function declaration; skipping function body