C++ How to write a friend function binary operator which lives in a namespace in a separate compilation unit - c++17

Question

I can't seem to figure out how to get the following to compile.
I am trying to write a binary operator which:
Is defined in a separate compilation unit (file)
Lives in a nested namespace
Here's some code:
// header.h
namespace ns1
{
namespace ns2
{
class myClass
{
friend bool operator==(const myClass& l, const myClass& r);
protected:
int a;
};
}
}
// header.cpp
#include "header.h"
using namespace ns1;
using namespace ns1::ns2;
bool operator==(const myClass& l, const myClass& r)
{
if(l.a != r.a) return false;
return true;
}
// main.cpp
#include "header.h"
using namespace ns1::ns2;
using namespace std;
int main()
{
myClass class1;
myClass class2;
if(class1 == class2)
{
cout << "hello world" << endl;
}
return 0;
}
This is the output from the compiler:
In function ‘bool operator==(const ns1::ns2::myClass&, const ns1::ns2::myClass&)’:
error: ‘int ns1::ns2::myClass::a’ is protected within this context
I have a suspicion that this is related to the compiler not understanding which namespace operator== should be in. I have tried explicitly declaring this, but that didn't help either.
The question is what is going on here? What is the compiler thinking?
Edit
Note: I posted this edit in response to an answer which was then deleted. The suggestion was to put the operator inside ns1::ns2, however this did not work. See the below output.
New compiler output:
error: ‘bool ns1::ns2::operator==(const ns1::ns2::myClass&, const ns1::ns2::myClass&)’ has not been declared within ‘ns1::ns2’ [-Werror]
bool ns1::ns2::operator==(const myClass& l, const myClass& r)
note: only here as a ‘friend’
friend bool operator==(const myClass& l, const myClass& r);

Answer 1

The problem here is when you declare a friend function inside your class, this function belongs to the innermost enclosing namespace, you have to define
bool ns1::ns2::operator==(const myClass& l, const myClass& r)
It should be defined inside namespace ns1::ns2 but not just introduced it with using directives,
// header.cpp
#include "header.h"
namespace ns1 {
namespace ns2 {
bool operator==(const myClass& l, const myClass& r)
{
if(l.a != r.a) return false;
return true;
}
}
}
Demo
or
// header.h
namespace ns1
{
namespace ns2
{
class myClass
{
...
};
bool operator==(const myClass& l, const myClass& r);
}
}
// header.cpp
bool ns1::ns2::operator==(const myClass& l, const myClass& r)
Demo
Another way is to declare your friend function as a global one,
// header.h
#pragma once
namespace ns1
{
namespace ns2
{
class myClass;
}
}
bool operator==(const ns1::ns2::myClass& l, const ns1::ns2::myClass& r);
namespace ns1
{
namespace ns2
{
class myClass
{
friend bool ::operator==(const myClass& l, const myClass& r);
protected:
int a;
};
}
}
// header.cpp
#include "header.h"
using namespace ns1::ns2;
bool operator==(const myClass& l, const myClass& r)
{
if(l.a != r.a) return false;
return true;
}
Demo