[Bug c/59520] a possible inconsistency in error diagnostics with "-pedantic -std=c99"

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59520

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |jakub at gcc dot gnu.org

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
> Let me raise another question that is unrelated, but perhaps you folks, in
> particular Joseph, could help add some clarify as I have been baffled by a
> couple of examples. 
> 
> In particular, are the following well-defined according the standard or they
> have undefined behavior? 

AFAIK neither is well-defined according to base standards, in C (and C++?) the
only valid accesses to union are to the last stored field, but it is accepted
as GNU extension.  

> Ex 2
> ====
> 
> int printf (const char *, ...);
> 
> struct S0
> {
>   char f0;
>   int f1;
> };
> 
> union
> {
>   int f0;
>   struct S0 f1;
> } d;
> 
> int
> main ()
> {
>   struct S0 g = {0,0};
>   d.f1 = g;
>   printf ("%d\n", d.f0);

This is of course undefined behavior even with the GNU extensions, padding bits
in g are undefined and you are then accessing them.  It is the same thing
as if you did struct S0 g = {0,0}; memcpy (&d.f0, &g, sizeof (int)); printf
("%d\n", d.f0);