[Bug c/80454] New: -Wmissing-braces wrongly warns about universal zero initializer {0}

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80454

            Bug ID: 80454
           Summary: -Wmissing-braces wrongly warns about universal zero
                    initializer {0}
           Product: gcc
           Version: 6.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

Consider:

typedef union { char c; int i; } union_t;

struct { struct { int a; long b; } x; int y; } t = { { 0 }, 1 };

struct { struct { union_t a; long b; } x; int y; } u = { { 0 }, 1 };

As expected, I do not get a warning for the first initialization. But with
Debian's GCC 6.3.0 20170415 (and previous versions GCC 4.9 and 5), I get a
warning for the second one:

tst.c:5:56: warning: missing braces around initializer [-Wmissing-braces]
 struct { struct { union_t a; long b; } x; int y; } u = { { 0 }, 1 };
                                                        ^
tst.c:5:56: note: (near initialization for ‘u’)

(pthread_rwlock_t from <pthread.h> is such a union type on my machine, so that
there's the same problem with this.)

It seems that not everything was fixed in PR 53119.