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

[Wolfgang Bangerth <bangerth@ices.utexas.edu>]

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

From: Wolfgang Bangerth <bangerth@ices.utexas.edu>
To: gcc-gnats@gcc.gnu.org
Cc:  
Subject: Re: c++/10437: "using namespace" at global scope creates incorrect
 code (fwd)
Date: Fri, 25 Apr 2003 14:15:09 -0500 (CDT)

 ---------- Forwarded message ----------
 Date: 22 Apr 2003 11:59:01 -0000
 From: Dean Foster <foster@diskworld.wharton.upenn.edu>
 To: bangerth@ices.utexas.edu
 Subject: Re: c++/10437: "using namespace" at global scope creates incorrect
     code
 
 
 I've shortened it down to about 50 lines.  (Took several 100 compiles.
 First to generate the error then move the one line, and confirm the
 error goes away!)  It is cut-and-pasted at the end of this email.
 
 There are several lines that turned out to be needed in order for the
 error to be generated.  The line ** 1 ** actually generates the error.
 The line ** 2 ** calls the error_generator.  Line ** 3 ** is where the
 error is.  It shouldn't ever try the apl::operator-() code, but it
 does if the "using namespace" is outside of the main().
 
 Move line ** 4 ** around to see the most obvious way that this is an
 error. 
 
 Thanks for your willingness to be patient.  Of course putting "using
 namespace"'s at global scope is pure evil!  We only had it in our test
 code.  
 
 later,
 
 dean
 
 
 =============================================================================
 Dean Foster                                                   dean@foster.net
 Statistics, Wharton, U. Penn                                     215 898 8233
 Philadelphia PA 19104-6340                 http://diskworld.wharton.upenn.edu
 =============================================================================
 
 // $Id: namespace_error.cc,v 4.29 2003/04/22 11:45:10 foster Exp $  
 
 #include <utility>
 
 class Iterator
 {
 public:
   void operator-(Iterator){};
 };
  
 template < class T>
 class flag_error 
 {
 public:
   typedef typename T::type_doesnt_exist unused_type;                   // ** 1 **
   typedef std::pair<T,T> pair;
 };
 
 namespace apl
 {  
   template <class T>
   typename flag_error<T>::pair                                          // ** 2 **
   operator-(const T& ,  const T& )
   {
   }
 } 
 
 std::pair<Iterator,Iterator>
 pair_iterator_constructor()
 {
 };
 
 template<class T>
 void
 foo(T t)
 {
   t.first - t.second;                                                   // ** 3 **
 }
 
 //  moving ** 4 ** from inside to outside of the main() changes from "legal" to "errors"
 
 using namespace apl;                                                    // ** 4 **
 int main()
 { 
   foo(pair_iterator_constructor());
 }