namespace_globals_problem.cpp

/** a.h */
#include "f_decl.h" // here're forward declrs
#include "b.h"      // needed for `Aa` member. Can't be incomplete type.
namespace A
{
    const int a = 10;
    const int b = 200;
    class Aa
    {
        Bb b;       // must have definition
    };
}

/** b.h */
#include "f_decl.h"
// #include "a.h" include that doesn't solve issue
namespace B
{
    class Bb
    {
        std::array< int, A::a > b_member; // error, compler doesn't see `a` and/or `b` in `A` namespace
        auto foo() -> void;
    };
}

/** b.cpp */
#include "b.h"
#include "f_decl.h"
auto B::Bb::foo() -> void
{
     A::a; // compile error
     B::b; // compile error
}

/** f_decl.h */
// some stuff like `class foo; struct foo1;` etc.
namespace A
{
    extern const int a; // ?
    extern const int b; // ?
    class Aa;
}

namespace B
{
    class Bb;
}