Re: c++/10437: "using namespace" at global scope creates incorrect code

[Dean Foster <foster@diskworld.wharton.upenn.edu>]

The following reply was made to PR c++/10437; it has been noted by GNATS.

From: Dean Foster <foster@diskworld.wharton.upenn.edu>
To: bangerth@dealii.org, dean@foster.net, gcc-bugs@gcc.gnu.org,
  gcc-prs@gcc.gnu.org, nobody@gcc.gnu.org, gcc-gnats@gcc.gnu.org
Cc:  
Subject: Re: c++/10437: "using namespace" at global scope creates incorrect code
Date: 21 Apr 2003 19:18:52 -0000

 Sorry about forcing you to do more of the reduction.  I did at least
 remove several 1000 lines of code!  But, every time I tried reducing
 it further, I ended up removing the problem.  Thanks.
 
 I'm willing to agree that that I've have an error/ambiguity in my
 code.  But I don't understand why bringing in a namespace that isn't
 being called should change the ambiguity status.  In other
 words,
 
 		main()
 		{
 		  using namespace foo;
 		  /* ... */           // no error
 		}
 doesn't generate an error.  Neither does:
 
 		main()
 		{
 		  /* ... */          // no error
 		}
 
 But, the following does generate an error:
 		  
 		using namespace foo;
 		main()
 		{
 		  /* ... */         // generates error
 		}
 
 It seems that the first and the 3rd should be the same.  So maybe the
 problem is that the third one is correct and that the first two should
 catch the ambigity?  Or is my understanding of namespaces incorrect?
 
 
 later,
 
 dean
 
 
 =============================================================================
 Dean Foster                                                   dean@foster.net
 Statistics, Wharton, U. Penn                                     215 898 8233
 Philadelphia PA 19104-6340                 http://diskworld.wharton.upenn.edu
 
 
 > Date: 21 Apr 2003 15:06:07 -0000
 > From: bangerth@dealii.org
 > Reply-To: bangerth@dealii.org, dean@foster.net, gcc-bugs@gcc.gnu.org,
 >   gcc-prs@gcc.gnu.org, nobody@gcc.gnu.org, gcc-gnats@gcc.gnu.org
 > X-SBRule: Spammy To: Header
 > X-SBRule: Other Spammy Headers
 > 
 > Synopsis: "using namespace" at global scope creates incorrect code
 > 
 > State-Changed-From-To: open->closed
 > State-Changed-By: bangerth
 > State-Changed-When: Mon Apr 21 15:06:06 2003
 > State-Changed-Why:
 >     I don't think this is a bug. After quite some reducing,
 >     your code looks like this:
 >     ---------------------------
 >     struct X {
 >         int operator-(const X& it);
 >     };
 >     
 >     template <class T>
 >     struct Y {
 >         typedef typename T::SOME_TYPE SOME_TYPE;
 >         typedef int type;
 >     };
 >     
 >     #ifdef BUG
 >     template <class T> typename Y<T>::type operator-(T,T);
 >     #endif
 >     
 >     int x = X() - X();
 >     -----------------------------
 >     (I replaced your using directive by the conditional
 >     statement -- both just bring into visibility the
 >     operator-.)
 >     
 >     What happens is that in the minus statement (which was
 >     inside a std:: function in your example), both possible
 >     operator- are considered, i.e. the member function and
 >     the global one. The global one requires instantiation
 >     of Y, at which point we realize that X::SOME_TYPE does
 >     not exist:
 >     g/x> /home/bangerth/bin/gcc-3.4-pre/bin/c++ -c x.cc -DBUG
 >     x.cc: In instantiation of `Y<X>':
 >     x.cc:15:   instantiated from here
 >     x.cc:7: error: no type named `SOME_TYPE' in `struct X'
 >     
 >     Tracing back where this happens in your code is a little
 >     more complicated, but I guess this is what happens -- you
 >     try to use __gnu_cxx::normal_iterator::const_iterator
 >     when bringing into scope the function in the namespace,
 >     but this const_iterator doesn't exist.
 >     
 >     Wolfgang
 > 
 > http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=10437
 >