Can I avoid nested namespaces in forward declaration?

Question

As of January 2018, what is the most modern style for forward declarations? We use something like below, but is there a more compact version?

Could I write something like class Foo::Bar (does not work for me, but maybe I miss a newer C++ standard)? Or any more compact version?

Current style:

namespace Foo
{
    class SimpleCommandParser;

    namespace Bar
    {
        class Parts;
        class Situation;
        class Callsign;
    }
    namespace Yep
    {
        class AircraftSnapshot;
        class Aircraft;
    }
}

There are some questions about similar topics like Forward Declaration in a nested namespace , but they are quite old.

Answer 1

As of January 2018, forward declaration of a name into a namespace is still the same as it was since first version of the standard i.e. the declaration must be within that namespace.

Could I write something like class Foo::Bar

Not in a current (C++17) standard.

There has been a proposal P0289R0 to add just such feature, but it hasn't been followed through and such change hasn't been introduced to the standard as of C++17.