[Bug c++/103534] [12 regression] Spurious -Wstringop-overflow warning with std::string concatencation

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103534

--- Comment #4 from Martin Sebor <msebor at gcc dot gnu.org> ---
Yes, the warning does disappear when malloc() and free() are used instead of
operator new and delete.  foo() also ends up much better optimized, even at
-O1:

__attribute__((abi_tag ("cxx11")))
struct string foo ()
{
  struct string & _7(D);
  char * _69;

  <bb 2> [local count: 1073741824]:
  _69 = __builtin_malloc (17);
  __builtin_memcpy (_69, "1234567890123456", 16);
  MEM[(char_type &)_69 + 16] = 0;
  MEM[(struct basic_string *)_7(D)]._M_dataplus._M_p = _69;
  MEM[(struct basic_string *)_7(D)].D.33183._M_allocated_capacity = 16;
  MEM[(struct basic_string *)_7(D)]._M_string_length = 16;
  return _7(D);

}

The -O1 dump in comment #1 doesn't look right, I may have messed something up. 
The same issue happens at -O2 where the dump is as follows:

=========== BB 3 ============
Imports: n_5(D)  
Exports: _1  n_5(D)  
         _1 : n_5(D)(I)  
n_5(D)  int [-INF, -1][1, +INF]
    <bb 3> [local count: 536870913]:
    _1 = (sizetype) n_5(D);
    if (_1 == 1)
      goto <bb 4>; [51.12%]
    else
      goto <bb 5>; [48.88%]

_1 : sizetype [1, 2147483647][18446744071562067968, +INF]
3->4  (T) _1 :  sizetype [1, 1]
3->4  (T) n_5(D) :      int [1, 1]
3->5  (F) _1 :  sizetype [2, 2147483647][18446744071562067968, +INF]
3->5  (F) n_5(D) :      int [-INF, -1][2, +INF]

=========== BB 4 ============
    <bb 4> [local count: 274448412]:
    MEM[(char *)&b] = 0;
    goto <bb 6>; [100.00%]


=========== BB 5 ============
    <bb 5> [local count: 262422500]:
    __builtin_memcpy (&b, &a, _1);