-stringop-truncation works as expected with statically allocated arrays but fails to detect the same problem involving dynamically allocated arrays.

$ cat t.c && gcc -O2 -S -Wall t.c
void sink (void*);

void f0 (const char *s)
{
  char a[7];
  __builtin_strncpy (a, s, sizeof a);   // -Wstringop-truncation (good)
  sink (a);
}

void f1 (const char *s, unsigned n)
{
  if (n > 256)
    n = 256;

  char a[n];
  __builtin_strncpy (a, s, n);   // missing warning

  sink (a);
}

void f2 (const char *s, unsigned n)
{
  if (n > 256)
    n = 256;

  char *p = __builtin_alloca (n);   // missing warning

  __builtin_strncpy (p, s,  n);
  sink (p);
}

void f3 (const char *s, unsigned n)
{
  if (n > 256)
    n = 256;

  char *p = __builtin_malloc (n);

  __builtin_strncpy (p, s, n);   // missing warning
  sink (p);
}

t.c: In function ‘f0’:
t.c:6:3: warning: ‘__builtin_strncpy’ specified bound 7 equals destination size [-Wstringop-truncation]
   __builtin_strncpy (a, s, sizeof a);   // -Wstringop-truncation (good)
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~