https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85205
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Mijzelf from comment #0)
> args.data[ i ].var1 = i;
This is undefined behaviour when i>0 because the array only has one element.
It's irrelevant that the union has additional space, the array doesn't.
> If the code is changed so data has 0 elements:
>
> struct X{
> int length;
>
> struct Y {
> int var1;
> int var2;
> } data[ 0 ];
> };
>
> the code works as expected
This works due to a GNU extension:
https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html#Zero-Length
