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

[sss@li-snyder.org]

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

            Bug ID: 103534
           Summary: [12 regression] Spurious -Wstringop-overflow warning
                    with std::string concatencation
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sss@li-snyder.org
  Target Milestone: ---

hi -

With a recent checkout of gcc12 (20211201) on a x86_64-pc-linux-gnu host,
compiling the following source with -O --std=c++20 gives a bogus
-Wstringop-overflow warning:

-- x.cc ---------------------------------------------------
#include <string>

std::string foo()
{
  return std::string("1234567890123456")  + std::string("");
}
-----------------------------------------------------------

$ g++ -c -O --std=c++20 x.cc
In file included from /usr/local/gcc/include/c++/12.0.0/string:40,
                 from x.cc:1:
In static member function ‘static constexpr std::char_traits<char>::char_type*
std::char_traits<char>::copy(std::char_traits<char>::char_type*, const
std::char_traits<char>::char_type*, std::size_t)’,
    inlined from ‘static constexpr void std::__cxx11::basic_string<_CharT,
_Traits, _Alloc>::_S_copy(_CharT*, const _CharT*,
std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type) [with _CharT =
char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ at
/usr/local/gcc/include/c++/12.0.0/bits/basic_string.h:423:21,
    inlined from ‘constexpr std::__cxx11::basic_string<_CharT, _Traits,
_Allocator>& std::__cxx11::basic_string<_CharT, _Traits,
_Alloc>::_M_append(const _CharT*, std::__cxx11::basic_string<_CharT, _Traits,
_Alloc>::size_type) [with _CharT = char; _Traits = std::char_traits<char>;
_Alloc = std::allocator<char>]’ at
/usr/local/gcc/include/c++/12.0.0/bits/basic_string.tcc:417:19,
    inlined from ‘constexpr std::__cxx11::basic_string<_CharT, _Traits,
_Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::append(const
std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char;
_Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ at
/usr/local/gcc/include/c++/12.0.0/bits/basic_string.h:1385:25,
    inlined from ‘constexpr std::__cxx11::basic_string<_CharT, _Traits,
_Allocator> std::operator+(std::__cxx11::basic_string<_CharT, _Traits,
_Allocator>&&, std::__cxx11::basic_string<_CharT, _Traits, _Allocator>&&) [with
_CharT = char; _Traits = std::char_traits<char>; _Alloc =
std::allocator<char>]’ at
/usr/local/gcc/include/c++/12.0.0/bits/basic_string.h:3530:23,
    inlined from ‘std::string foo()’ at x.cc:5:59:
/usr/local/gcc/include/c++/12.0.0/bits/char_traits.h:426:56: warning: ‘void*
__builtin_memcpy(void*, const void*, long unsigned int)’ specified bound
between 18446744073709551600 and 18446744073709551615 exceeds maximum object
size 9223372036854775807 [-Wstringop-overflow=]
  426 |         return static_cast<char_type*>(__builtin_memcpy(__s1, __s2,
__n));
      |                                       
~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
$ 



Perhaps interestingly, the warning goes away if the first string literal
is less than 16 characters long.