So while that warning can certainly be useless information, it does definitely catch bugs. Unfortunately, the people who write code like the above are also more likely to be people who ignore warnings. ;-)
#include <string.h>
struct S {
int i;
char c;
int j;
};
void f(void) {
struct S s1 = { 0, 0, 0 }, s2 = { 0, 0, 0 };
memcpy(&s1, &s2, 9);
}
int main(void) {
f();
return 0;
}