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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/104965] New: Yet another -Warray-bounds false positive
Date: Thu, 17 Mar 2022 12:15:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104965

            Bug ID: 104965
           Summary: Yet another -Warray-bounds false positive
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
            Blocks: 56456
  Target Milestone: ---

Maybe another dup, I can't keep track.

#include <string>

template<typename T>
T* f(const std::basic_string<T>& str)
{
  auto n =3D str.size();
  auto p =3D new T[n];
  str.copy(p, n);
  return p;
}

int main()
{
  std::basic_string<unsigned short> s;
  auto p =3D f(s);
  char c =3D 0;
  if (s.size())
    c =3D *p;
  delete[] p;
  return c;
}


With -O2

copy.cc: In function 'int main()':
copy.cc:18:9: warning: array subscript 0 is outside array bounds of 'short
unsigned int [0]' [-Warray-bounds]
   18 |     c =3D *p;
      |         ^~
In function 'T* f(const std::__cxx11::basic_string<_CharT>&) [with T =3D sh=
ort
unsigned int]',
    inlined from 'int main()' at copy.cc:15:13:
copy.cc:7:12: note: object of size 0 allocated by 'operator new []'
    7 |   auto p =3D new T[n];
      |            ^~~~~~~~


This is ridiculous. The array subscript is guarded by the same length as the
array. GCC manages to use the string length to determine the allocation siz=
e,
but can't use it to confirm the conditional read doesn't happen.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D56456
[Bug 56456] [meta-bug] bogus/missing -Warray-bounds=