With the following code, I get the message 'namespace_parent.h' does not need to be #included, which is incorrect.

parent.h

namespace parent
{
   class Parent {};
}

child.h

#include "namespace_parent.h"

using namespace parent;

namespace child
{
   class Child : Parent {};
}

The code compiles correctly but 'using namespace' in a header file is bad practice, so it might be better to give a warning that it is better to use full namespaces.