The order of evaluation among initializers is unspecified, which makes it UB.

We don't have to use mixed-up designated initializers to run aground. Just simply:

  { int a = 0;
    int x[2] = { a++, a++ }; }

This was also new in C99 (not to mention the struct literal syntax); in C90, run-time values couldn't be used for initializing aggregate members, so the issue wouldn't arise.

Code like:

   { int a = 0;
     struct foo f = { a, a }; }

was supported in the GNU C dialect of C90 before C99 and of course is also a long-time C++ feature.

https://gcc.gnu.org/onlinedocs/gcc/Initializers.html#Initial...

The doc doesn't say anything about order. I can't remember what C++ has to say about this.