This testcase from an IBM internal user:
                                                                                
  #include <iostream>
  class mytest {
  public:
    mytest (int);
    unsigned char operator|| (const mytest&) const;
    friend unsigned char operator|| (const int&, const mytest&);
  };
  inline unsigned char operator|| (const int& lhs, const mytest& rhs)
  {
    std::cout << 1 << std::endl;
    return (mytest)lhs || rhs;
  }
                                                                                
fails with FSF 4.0 releases and the current 4.0 branch with:
                                                                                
elm3b11% /opt/gcc-nightly/4.0/bin/g++ -c bug.cc
/home/gcc-nightly/4.0-20050913/bin/../lib/gcc/powerpc64-linux/4.0.2/../../../../include/c++/4.0.2/ostream:
In member function ‘std::basic_ostream<_CharT, _Traits>&
std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char,
_Traits = std::char_traits<char>]’:
bug.cc:10:   instantiated from here
/home/gcc-nightly/4.0-20050913/bin/../lib/gcc/powerpc64-linux/4.0.2/../../../../include/c++/4.0.2/ostream:194:
error: ISO C++ says that these are ambiguous, even though the worst conversion
for the first is better than the worst conversion
for the second:
/home/gcc-nightly/4.0-20050913/bin/../lib/gcc/powerpc64-linux/4.0.2/../../../../include/c++/4.0.2/ostream:194:
note: candidate 1: operator||(bool, bool) <built-in>
bug.cc:8: note: candidate 2: unsigned char operator||(const int&, const mytest&)
                                                                               
This is due to code for operator<<(int) in std_ostream.h that uses || on
integral (not bool) operands, leading to the ambiguity between the built-in
|| operator and one defined in the testcase.  It is a regression because
versions earlier than 4.0 failed to report the error.
                                                                                
The testcase passes on mainline starting with this patch from Paolo Carlini:
                                                                                
  http://gcc.gnu.org/ml/gcc-cvs/2005-07/msg00424.html
  http://gcc.gnu.org/ml/gcc-patches/2005-07/msg00726.html
                                                                                
I'm currently testing this part of that patch with the 4.0 branch:

        * include/std/std_ostream.h (operator<<(short), operator<<(int)):
        Adjust logic, as per the letter of the resolution of DR117 [WP].