Re: build error --std=c++17 vs --std=c++23 -Wrestrict "may overlap up to 9223372036854775813 bytes at offset -3"

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Sat, 11 Feb 2023, 23:38 Jonny Grant, <jg@jguk.org> wrote:

> Hello
>
> Anyone else seeing an issue when using c++23 ?
> gcc (Ubuntu 12.2.0-3ubuntu1) 12.2.0
>
> The only change is to use --std=c++23
>

With C++17 std::string is instantiated in the library, not in your code.
With C++20 your code instantiates it. That's the only reason the -std
option matters here.





> -Wrestrict is enabled by -Wall
>
>
> #include <string>
>
> typedef struct a_bc
> {
>     std::string a;
>     std::string b;
> } a_t;
>
> void f()
> {
>     a_t c;
>
>     c.a = " sdfsdf fsdfsdf fdfsfdsdf ";   // seems to need this long
> string to reproduce, down to 8 bytes it didn't
>     c.b = "E";
> }
>
>
>
>
>
>
>
>
> $ g++ -O3 --std=c++23 -Wall -c -o gcc_err3 src/gcc_err3.cpp
> In file included from /usr/include/c++/12/string:40,
>                  from src/gcc_err3.cpp:4:
> 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>::assign(const
> _CharT*) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc =
> std::allocator<char>]’ at /usr/include/c++/12/bits/basic_string.h:1647:19,
>     inlined from ‘constexpr std::__cxx11::basic_string<_CharT, _Traits,
> _Alloc>& std::__cxx11::basic_string<_CharT, _Traits,
> _Alloc>::operator=(const _CharT*) [with _CharT = char; _Traits =
> std::char_traits<char>; _Alloc = std::allocator<char>]’ at
> /usr/include/c++/12/bits/basic_string.h:815:28,
>     inlined from ‘void f(const std::string&, bool, bool)’ at
> src/gcc_err3.cpp:17:13:
> /usr/include/c++/12/bits/char_traits.h:431:56: warning: ‘void*
> __builtin_memcpy(void*, const void*, long unsigned int)’ accessing
> 9223372036854775810 or more bytes at offsets -4611686018427387902 and
> [-4611686018427387903, 4611686018427387904] may overlap up to
> 9223372036854775813 bytes at offset -3 [-Wrestrict]
>   431 |         return static_cast<char_type*>(__builtin_memcpy(__s1,
> __s2, __n));
>       |
> ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
>