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From: "prasoonsaurav.nit at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/51836] New: -Wsequence-point fails when convoluted expressions with multiple side effects are used (C++03)
Date: Thu, 12 Jan 2012 09:11:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51836

             Bug #: 51836
           Summary: -Wsequence-point fails when convoluted expressions
                    with multiple side effects are used (C++03)
    Classification: Unclassified
           Product: gcc
           Version: 4.4.6
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: prasoonsaurav.nit@gmail.com


Consider the following example


int main()
{ 
      int i=10;
      i += (i , i++, i) + i; // also invokes UB
}


prasoon@Prasoon:~/test_code$ cat ub.cpp
int main()
{ 
      int i=10;
      i += (i , i++, i) + i; // also invokes UB
}
prasoon@Prasoon:~/test_code$ g++ -Wsequence-point ub.cpp
prasoon@Prasoon:~/test_code$ 

I don't get any warning like 'operation on 'i' may be undefined.

Another similar example

int main()
{
    char *str;
    char array[100]= "Hello";
    if((str = array)[0] == 'H'){
         //do something
    }
}

As per my understanding (str = array)[0] also invokes UB but no warning is
given by g++ even after using that option.

Another example is 

int main()
{
   int a = 10;
   ++a = 100; // UB
}