https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116063
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Hubert Tong from comment #2)
> If it is invalid, a diagnostic like this should appear:
> ```
> warning: union cannot be made transparent
> ```
Not always.
typedef union __attribute__((__transparent_union__)) U {
unsigned int u;
int x;
} U;
also fails the same way because signed int and unsigned int are passed
differently.
