Bogus -Wrestrict in builtin_memcpy from std::string & C++20 on kinetic

[Chris Halse Rogers]

The attached cpp file, when built with `g++ -O3 -S --std=c++20 -o test.o gcc-bug-test.cpp -Wall -Werror` on g++12, fails to compile with the following error. Notably, this error requires *both* --std=c++20 and -O3 - it compiles correctly at -O2, and it compiles correctly at -O3 with --std=c++17.

In file included from /usr/include/c++/12/string:40,
                 from gcc-bug-test.cpp:1:
In static member function ‘static constexpr std::char_traits<char>::char_type* std::char_traits<char>::copy(char_type*, const char_type*, std::size_t)’,
    inlined from ‘static constexpr void std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_S_copy(_CharT*, const _CharT*, size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ at /usr/include/c++/12/bits/basic_string.h:423:21,
    inlined from ‘constexpr std::__cxx11::basic_string<_CharT, _Traits, _Allocator>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_M_replace(size_type, size_type, const _CharT*, size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ at /usr/include/c++/12/bits/basic_string.tcc:532:22,
    inlined from ‘constexpr std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::replace(size_type, size_type, const _CharT*, size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ at /usr/include/c++/12/bits/basic_string.h:2171:19,
    inlined from ‘constexpr std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::insert(size_type, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ at /usr/include/c++/12/bits/basic_string.h:1858:29,
    inlined from ‘constexpr std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(__cxx11::basic_string<_CharT, _Traits, _Allocator>&&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&) [with _CharT = char; _Traits = char_traits<char>; _Alloc = allocator<char>]’ at /usr/include/c++/12/bits/basic_string.h:3531:35,
    inlined from ‘std::string to_camel_case(const std::string&)’ at gcc-bug-test.cpp:6:45:
/usr/include/c++/12/bits/char_traits.h:431:56: error: ‘void* __builtin_memcpy(void*, const void*, long unsigned int)’ accessing 9223372036854775810 or more bytes at offsets [2, 9223372036854775807] and 1 may overlap up to 9223372036854775813 bytes at offset -3 [-Werror=restrict]
  431 | return static_cast<char_type*>(__builtin_memcpy(__s1, __s2, __n));
      | ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors

[[Testcase]]
$ cat gcc-bug-test.cpp

#include <string>

std::string to_camel_case(std::string const& name)
{
    std::string camel_cased_name;
    camel_cased_name = std::string{name[0]} + name.substr(1);
    return camel_cased_name;
}

$ g++ -O3 -S --std=c++20 -o test.o gcc-bug-test.cpp -Wall -Werror

ProblemType: Bug
DistroRelease: Ubuntu 22.10
Package: g++ 4:12.2.0-1ubuntu1
ProcVersionSignature: Ubuntu 5.19.0-15.15-generic 5.19.0
Uname: Linux 5.19.0-15-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.23.0-0ubuntu2
Architecture: amd64
CasperMD5CheckResult: pass
Date: Mon Sep 26 16:02:09 2022
InstallationDate: Installed on 2021-06-26 (456 days ago)
InstallationMedia: Ubuntu 21.10.0 2021.05.28 amd64 "bcachefs" (20210622)
SourcePackage: gcc-defaults
UpgradeStatus: Upgraded to kinetic on 2022-09-13 (13 days ago)