[Bug c/94558] New: Designated initializer inside _Generic is misinterpreted

["elronnd at elronnd dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94558

            Bug ID: 94558
           Summary: Designated initializer inside _Generic is
                    misinterpreted
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: elronnd at elronnd dot net
  Target Milestone: ---

POC:

typedef struct{int _;} A;
typedef struct{int _;} B;

typedef union {
        A a;
        B b;
} O;

#define NO(o) _Generic((o), \
        A: ((O){.a=o}), \
        B: ((O){.b=o}))

int main() {
        A a;
        O o = NO(a);
}


Generates the error message:

incompatible types when initializing type ‘int’ using type ‘A’ {aka ‘struct
<anonymous>’}

Because it's trying to initialize the '_' field in 'A' with 'a', instead of the
'a' field in 'O'.