[Bug 1990811] Re: Bogus -Wrestrict in builtin_memcpy from std::string & C++20

[Michał Sawicz]

** Changed in: gcc-defaults (Ubuntu)
       Status: New => Confirmed

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

Status in gcc-defaults package in Ubuntu:
  Confirmed

Bug description:
  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

  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)

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