Unsequenced Side Effects on the Same Union

From	Shao Miller <sha0.miller@gmail.com>
Newsgroups	comp.std.c
Subject	Unsequenced Side Effects on the Same Union
Date	2013-01-15 22:32 -0500
Organization	A noiseless patient Spider
Message-ID	<kd570b$mvc$1@dont-email.me> (permalink)

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   #include <stdio.h>

   void func1(void * vp1, void * vp2) {
       int * ip;
       double * dp;

       /* Populate ip and dp? */
       if (!vp1 && !vp2) {
           func1(&ip, &dp);

           /*
            * 6.5p2: Unsequenced side effects
            * on the same... Scalar?
            */
           printf("%f\n", (*ip = 42) + (*dp = 3.14));
         }

       /* Self-called? */
       if (vp1 && vp2) {
           /* Aliasing shoved down here */
           static union {
               int i;
               double d;
             } u;
           *(int **) vp1 = &u.i;
           *(double **) vp2 = &u.d;
         }
     }

   int main(void) { func1(NULL, NULL); return 0; }

Sorry all, I'm puzzled...  This surely yields undefined behaviour, but 
why, in C11?  6.5p2 changed from that of C99.  Is it because one of the 
'+' operands could establish an effective type before the other? 
Hopefully they are not simultaneous.

-- 
- Shao Miller
--
"Thank you for the kind words; those are the kind of words I like to hear.

Cheerily,"  -- Richard Harter

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Unsequenced Side Effects on the Same Union Shao Miller <sha0.miller@gmail.com> - 2013-01-15 22:32 -0500
  Re: Unsequenced Side Effects on the Same Union Nick Bowler <nbowler@draconx.ca> - 2013-01-16 14:55 +0000
    Re: Unsequenced Side Effects on the Same Union Shao Miller <sha0.miller@gmail.com> - 2013-01-16 10:53 -0500
    Re: Unsequenced Side Effects on the Same Union James Kuyper <jameskuyper@verizon.net> - 2013-01-16 11:04 -0500
      Re: Unsequenced Side Effects on the Same Union Nick Bowler <nbowler@draconx.ca> - 2013-01-16 20:56 +0000
        Re: Unsequenced Side Effects on the Same Union Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-17 13:21 -0800
          Re: Unsequenced Side Effects on the Same Union Shao Miller <sha0.miller@gmail.com> - 2013-01-17 17:50 -0500
            Re: Unsequenced Side Effects on the Same Union Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-18 11:25 -0800
              Re: Unsequenced Side Effects on the Same Union Shao Miller <sha0.miller@gmail.com> - 2013-01-18 15:07 -0500

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