About using directives in namespace in header files

Question

(This is kind of related to a previous question.)

Core guideline SF.7 gives a good motivation for avoiding to put using namespace directives at global scope in a header file.

However, even writing using namespace in a non-global namespace can be problematic. For example:

• header1.hpp

  #include <boost/hana/transform.hpp>
  namespace ab {
      using namespace boost::hana;
      inline constexpr auto a = transform;
  }
  

• header2.hpp

  #include <range/v3/view/transform.hpp>
  namespace ab {
      using namespace ranges::views;
      inline constexpr auto b = transform;
  }
  

• main.cpp

  #include "header1.hpp"
  #include "header2.hpp"
  // here ab::b is ambiguous
  int main() {
  }
  

  with error (from clang)

  In included file: reference to 'transform' is ambiguous
  /home/enrico/header2.hpp:4:31:
  note: error occurred here
  /usr/include/boost/hana/fwd/transform.hpp:53:27:
  note: candidate found by name lookup is 'boost::hana::transform'
  :567:46:
  note: candidate found by name lookup is 'ranges::views::transform'
  

Is there a reason why the guidelines don't mention this?

Should we entirely avoid using namespace n;/using n::x;/namespace n = m; at namespace scope?

---

Related

In this question of mine I have self-answerd with a way to use those directives when defining an object in a header file, but without cluttering anything, by wrapping all RHS of assignment in a void-to-desired-object lambda called in place.