Extending std namespace with backported code

Question

Some background:

Extending namespace std is undefined-behaviour(UB) unless it is a template specialization [1]:

It is undefined behavior to add declarations or definitions to namespace std or to any namespace nested within std, with a few exceptions noted below

There also questions on SO which also say it's an UB and a bad idea [2, 3].

We use boost::tr1 on platforms that don't have tr1 support (e.g., WinCE, WM). Boost does exactly this: injects its own implementation into std::tr1 if tr1 is not provided.

For example in boost/tr1/memory.hpp:

namespace std{ namespace tr1{ 
   using ::boost::bad_weak_ptr;
   using ::boost::shared_ptr;
   ...
} }

My questions are:

• Does this mean that it is UB if boost::tr1 is used with non-tr1 compilers?

• Alternatively, is it okay to inject backports into std as long as developer makes sure that backported functionality is unavailable?

• If it is okay, can one take it one step further and inject boost::tr1 into std instead of std::tr1?