strange warning when importing template from namespace

[Simon Anders]

Hi,

the people at comp.lang.c++ advised me to rather ask this here:

I'm using the hash_set template from the GNU C++ standard library.
Because the hashed containers are not standard content of the library
but an extension taken over from SGI, the GNU people have put them into
a special namespace __gnu_cxx.

So I wrote:

#include <ext/hash_set>
using __gnu_cxx::hash_set;
[...]

Although this works fine, I get a compiler warning, namely:

stabilizer.h:12: Warning(315): Nothing known about
    '__gnu_cxx::hash_set'.

(with line 12 being the 'using' statement).

What is the compiler trying to warn me about, i.e. what would it like to
know? As all works fine, I wonder when it would not have worked.

TIA,
   Simon