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From: "elronnd at elronnd dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/94558] New: Designated initializer inside _Generic is
 misinterpreted
Date: Sat, 11 Apr 2020 07:49:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94558

            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=3Do}), \
        B: ((O){.b=3Do}))

int main() {
        A a;
        O o =3D NO(a);
}


Generates the error message:

incompatible types when initializing type =E2=80=98int=E2=80=99 using type =
=E2=80=98A=E2=80=99 {aka =E2=80=98struct
<anonymous>=E2=80=99}

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