[Bug c++/113239] New: [13 regression] After 822a11a1e64, bogus -Warray-bounds warnings in std::vector

["dimitry@unified-streaming.com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113239

            Bug ID: 113239
           Summary: [13 regression] After 822a11a1e64, bogus
                    -Warray-bounds warnings in std::vector
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dimitry@unified-streaming.com
  Target Milestone: ---

We noticed spurious warnings in some C++17 code compiled with g++ 13.2.0, and I
bisected it to commit 822a11a1e642e0abe92a996e7033a5066905a447 ("libstdc++: Do
not use memmove for 1-element ranges [PR108846]") for bug 108846 ("std::copy,
std::copy_n and std::copy_backward on potentially overlapping subobjects").

Reduced test case:

// g++ -std=c++17 -Wall -O2 -c testcase.cpp

#include <cstdint>
#include <vector>

struct frame_t
{
  uint64_t pts_;
  uint32_t timescale_;
  std::vector<uint8_t> data_;
};

struct frame_source_t
{
  virtual frame_t get() = 0;
};

struct frame_filter_t : frame_source_t
{
  frame_t get() override
  {
    if(current_frame_.data_.empty())
    {
      return current_frame_;
    }
    else
    {
      return frame_t();
    }
  }

  frame_t current_frame_;
};

frame_filter_t create_frame_filter()
{
  return frame_filter_t();
}

// EOT

With gcc-13-6371-ga41a56dee5c, this compiles without any warning. With
gcc-13-6372-g822a11a1e64 and later, up to gcc-14-6924-g00dea7e8c41 (master as
of 2024-01-04), you get:

In file included from
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/vector:62,
                 from testcase.cpp:5:
In static member function 'static void std::__copy_move<false, false,
std::random_access_iterator_tag>::__assign_one(_Tp*, _Up*) [with _Tp = unsigned
char; _Up = const unsigned char]',
    inlined from 'static _Up* std::__copy_move<_IsMove, true,
std::random_access_iterator_tag>::__copy_m(_Tp*, _Tp*, _Up*) [with _Tp = const
unsigned char; _Up = unsigned char; bool _IsMove = false]' at
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/stl_algobase.h:441:20,
    inlined from '_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove =
false; _II = const unsigned char*; _OI = unsigned char*]' at
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/stl_algobase.h:507:30,
    inlined from '_OI std::__copy_move_a1(_II, _II, _OI) [with bool _IsMove =
false; _II = const unsigned char*; _OI = unsigned char*]' at
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/stl_algobase.h:534:42,
    inlined from '_OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove =
false; _II = __gnu_cxx::__normal_iterator<const unsigned char*, vector<unsigned
char> >; _OI = unsigned char*]' at
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/stl_algobase.h:541:31,
    inlined from '_OI std::copy(_II, _II, _OI) [with _II =
__gnu_cxx::__normal_iterator<const unsigned char*, vector<unsigned char> >; _OI
= unsigned char*]' at
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/stl_algobase.h:637:7,
    inlined from 'static _ForwardIterator
std::__uninitialized_copy<true>::__uninit_copy(_InputIterator, _InputIterator,
_ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const
unsigned char*, std::vector<unsigned char> >; _ForwardIterator = unsigned
char*]' at
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/stl_uninitialized.h:147:27,
    inlined from '_ForwardIterator std::uninitialized_copy(_InputIterator,
_InputIterator, _ForwardIterator) [with _InputIterator =
__gnu_cxx::__normal_iterator<const unsigned char*, vector<unsigned char> >;
_ForwardIterator = unsigned char*]' at
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/stl_uninitialized.h:185:15,
    inlined from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator,
_InputIterator, _ForwardIterator, allocator<_Tp>&) [with _InputIterator =
__gnu_cxx::__normal_iterator<const unsigned char*, vector<unsigned char> >;
_ForwardIterator = unsigned char*; _Tp = unsigned char]' at
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/stl_uninitialized.h:373:37,
    inlined from 'std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp,
_Alloc>&) [with _Tp = unsigned char; _Alloc = std::allocator<unsigned char>]'
at
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/stl_vector.h:603:31,
    inlined from 'frame_t::frame_t(const frame_t&)' at testcase.cpp:7:8,
    inlined from 'virtual frame_t frame_filter_t::get()' at testcase.cpp:25:14:
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/stl_algobase.h:399:17:
warning: array subscript 0 is outside array bounds of 'unsigned char [0]'
[-Warray-bounds=]
  399 |         { *__to = *__from; }
      |           ~~~~~~^~~~~~~~~
In file included from
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/x86_64-pc-linux-gnu/bits/c++allocator.h:33,
                 from
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/allocator.h:46,
                 from
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/vector:63:
In member function '_Tp* std::__new_allocator<_Tp>::allocate(size_type, const
void*) [with _Tp = unsigned char]',
    inlined from 'static _Tp* std::allocator_traits<std::allocator<_Tp1>
>::allocate(allocator_type&, size_type) [with _Tp = unsigned char]' at
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/alloc_traits.h:475:28,
    inlined from 'std::_Vector_base<_Tp, _Alloc>::pointer
std::_Vector_base<_Tp, _Alloc>::_M_allocate(std::size_t) [with _Tp = unsigned
char; _Alloc = std::allocator<unsigned char>]' at
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/stl_vector.h:377:33,
    inlined from 'void std::_Vector_base<_Tp,
_Alloc>::_M_create_storage(std::size_t) [with _Tp = unsigned char; _Alloc =
std::allocator<unsigned char>]' at
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/stl_vector.h:395:44,
    inlined from 'std::_Vector_base<_Tp, _Alloc>::_Vector_base(std::size_t,
const allocator_type&) [with _Tp = unsigned char; _Alloc =
std::allocator<unsigned char>]' at
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/stl_vector.h:331:26,
    inlined from 'std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp,
_Alloc>&) [with _Tp = unsigned char; _Alloc = std::allocator<unsigned char>]'
at
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/stl_vector.h:600:61,
    inlined from 'frame_t::frame_t(const frame_t&)' at testcase.cpp:7:8,
    inlined from 'virtual frame_t frame_filter_t::get()' at testcase.cpp:25:14:
/home/dim/ins/gcc-14-6924-g00dea7e8c41/include/c++/14.0.0/bits/new_allocator.h:151:55:
note: object of size 0 allocated by 'operator new'
  151 |         return static_cast<_Tp*>(_GLIBCXX_OPERATOR_NEW(__n *
sizeof(_Tp)));
      |                                                       ^

I think these warnings might be bogus, since it seems to be complaining about
copying an empty std::vector<uint8_t> here?

It might also get confused due to the padding inserted between the
frame_t::pts_ and frame_t::timescale_ members, since if you remove either of
these members, the warning goes away.