[Bug c/65808] New: -pedantic -std=gnu11 results in warning for transparent_union usage

[equinox-gccbugs at diac24 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65808

            Bug ID: 65808
           Summary: -pedantic -std=gnu11 results in warning for
                    transparent_union usage
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: equinox-gccbugs at diac24 dot net

Created attachment 35359
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35359&action=edit
test program, compile with -std=gnu11 -pedantic

Trying to use a transparent_union results in a warning when -pedantic is used,
even though -std=gnu11 is specified.

Test program is this:
(gcc -std=gnu11 -pedantic -c /tmp/uniontest.c)

struct a { int a; };
struct b { int b; };

__extension__ union ab {
    struct a *pa;
    struct b *pb;
} __attribute__ ((transparent_union));

extern void func1 (union ab arg);
__extension__ extern void func2 (union ab arg);

void test(void)
{
    struct a sa = { 1 };
    func1(&sa);                 /* warning */
    __extension__ func1(&sa);   /* no warning */
    func2(&sa);                 /* warning */
}


Expected behaviour:  no warning at all, or some way to put __extension__ on the
union or the function prototype.
Actual behaviour: __extension__ must be used on each and every individual
function call, or a warning occurs:

/tmp/uniontest.c: In function ‘test’:
/tmp/uniontest.c:15:2: warning: ISO C prohibits argument conversion to union
type [-Wpedantic]
  func1(&sa);                 /* warning */
  ^
/tmp/uniontest.c:17:2: warning: ISO C prohibits argument conversion to union
type [-Wpedantic]
  func2(&sa);                 /* warning */
  ^