Created attachment 10633 [details]
A patch to fix

The simplest code (icmp6_test.c)

#include <stdlib.h>                                                           
#include <netinet/icmp6.h>                                                    
                                                                              
int main()                                                                    
{                                                                             
    struct icmp6_filter filter;                                               
                                                                              
    ICMP6_FILTER_SETPASS(ND_ROUTER_ADVERT, &filter);                          
                                                                              
    return EXIT_SUCCESS;                                                      
}

compiled with "gcc icmp6_test.c -Wsign-conversion" gives a warning

icmp6_test.c: In function ‘main’:
icmp6_test.c:8:2: warning: negative integer implicitly converted to unsigned type [-Wsign-conversion]
  ICMP6_FILTER_SETPASS(ND_ROUTER_ADVERT, &filter);