In <netinet/in.h>, we have the following definitions
(simplified for presentation):

/* IPv6 address */
struct in6_addr
  {
    union
      {
        uint8_t         u6_addr8[16];
        uint16_t        u6_addr16[8];
        uint32_t        u6_addr32[4];
        uint64_t        u6_addr64[2];
      } in6_u;
  };

#define IN6ADDR_ANY_INIT { 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 }
#define IN6ADDR_LOOPBACK_INIT { 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1 }

IN6ADDR_ANY_INIT and IN6ADDR_LOOPBACK_INIT are initializers
for static variables of the type struct in6_addr.  Because of
the nested struct/union/array definition, there need to be
three pairs of braces around the comma-separated constant
list.  The current definition has only one pair of braces and
hence causes compiler warnings.  A test program and compiler
output follow.

coffee:/u/wtc/tmp 101% cat in6addr.c
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

struct in6_addr any = IN6ADDR_ANY_INIT;
struct in6_addr loopback = IN6ADDR_LOOPBACK_INIT;

int main()
{
    return 0;
}
coffee:/u/wtc/tmp 102% gcc -Wall in6addr.c
in6addr.c:5: warning: missing braces around initializer for `any.in6_u'
in6addr.c:6: warning: missing braces around initializer for
`loopback.in6_u'