Forward declaration of class in different namespaces

Question

I already searched for questions about forward declaration but couldn't find a answer to my case.

I have a class Foo:

Foo.h

#pragma once

#include <string>

namespace aaa::xxx::yyy::zzz
{
    class Foo {
    public:
        Foo() = default;

        std::string get_value() const
        {
            return m_value;
        }

        void set_value(const std::string& value)
        {
            m_value = value;
        }
    private:
        std::string m_value{};
    };
}

This class Foo is used in annother class Bar:

Bar.h

#pragma once

namespace aaa::xxx::yyy::zzz 
{
    class Foo;
}

namespace aaa::bbb::ccc::ddd
{
    using Foo = aaa::xxx::yyy::zzz::Foo;

    class Bar 
    {
    public:
        Bar() = default;

        Foo get_foo() const;
    private:
        Foo m_foo{};
    };

}

Bar.cpp

#include "Bar.h"

#include "Foo.h"

namespace aaa::bbb::ccc::ddd
{
    using Foo = aaa::xxx::yyy::zzz::Foo;

    Foo Bar::get_foo() const
    {
        return m_foo;
    }

}   

Now i dont know what i did wrong with the forwad declaration. I declared class Foo before class Bar with its namespace but still its states that class Foo is undefined.

So the Question is how to forward class Foo correctly?

EDIT: Changing Foo m_foo{}; to Foo m_foo; as suggested still produces the same error.