For some reason the GCC # line directive suppresses warnings.  See bug 83404, comment 2 for the origin of this bug report.

$ cat a.c && gcc -O2 -S -Wall a.c
#include <string.h>

char x[12];

void f (void)
{
  __builtin_strncat (x, "aa", 1);   // warning (good)
}

void g (void)
{
  strncat (x, "aa", 1);   // no warning (bug)
}

a.c: In function ‘f’:
a.c:7:3: warning: ‘__builtin_strncat’ output truncated copying 1 byte from a string of length 2 [-Wstringop-truncation]
   __builtin_strncat (x, "aa", 1);   // warning (good)
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


The problem can be reproduced with another, simpler test case:

$ cat a.ii && gcc -O2 -S -Wall a.ii
char x[12];

void f (void)
{
  x[13] = 0;   // -Warray-bounds (good)
}

int g (void)
{
# 12 "a.c" 3 4
  return x[13];   // no warning (bug)
}
a.ii: In function ‘void f()’:
a.ii:5:7: warning: array subscript 13 is above array bounds of ‘char [12]’ [-Warray-bounds]
   x[13] = 0;   // -Warray-bounds (good)
   ~~~~^