[Bug tree-optimization/90905] missing -Wreturn-local-addr returning a local std::string::c_str()

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90905

--- Comment #6 from Martin Sebor <msebor at gcc dot gnu.org> ---
With str being a local (non-reference) variable this should be diagnosed
because of the str.D.28972._M_local_buf(12):

# _47 = PHI <_59(9), &str.D.28972._M_local_buf(12), _59(8)>
  str ={v} {CLOBBER};
  return _47;

In your example a is a reference argument but in this modified version:

  struct A { char *p; char c[13]; };

  void* f (struct A a, _Bool b)
  {
    a.p = b ? a.c : (char*)__builtin_malloc (13);
    __builtin_memcpy (a.p, "hello world!", 12);
    a.p[12] = 0;
    return a.p;
  }

and the IL:

  <bb 3> [local count: 354334802]:
  iftmp.0_7 = __builtin_malloc (13);

  <bb 4> [local count: 1073741824]:
  # iftmp.0_2 = PHI <iftmp.0_7(3), &a.c(2)>
  a.p = iftmp.0_2;
  __builtin_memcpy (iftmp.0_2, "hello world!", 12);
  _1 = a.p;
  MEM[(char *)_1 + 12B] = 0;
  return _1;

the only challenge with detecting the bug that I see is making a record of the
rhs of the assignment to _1 = a.p (and others like that) and then checking the
prior assignment to a.p (et al.).  With that in place the "may return" warning
will trigger.