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From: "dangelog at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/108846] std::copy, std::copy_n and std::copy_backward
  on potentially overlapping subobjects
Date: Fri, 24 Feb 2023 19:31:49 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108846

--- Comment #12 from Giuseppe D'Angelo <dangelog at gmail dot com> ---
(In reply to Jonathan Wakely from comment #9)
> (In reply to Giuseppe D'Angelo from comment #5)
> > https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/=
bits/
> > stl_algobase.h#L417-L437
> >=20
> > Is the extent of the fix just to add another branch to the if (_Num) ch=
eck
> > here?
>=20
> Yes, I think that's the simplest place to fix it.
>=20
> And we can even get rid of the static assertion, because the Num=3D=3D1 b=
ranch
> will require assignability now.
>=20
> --- a/libstdc++-v3/include/bits/stl_algobase.h
> +++ b/libstdc++-v3/include/bits/stl_algobase.h
> @@ -737,16 +737,11 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
>         static _Tp*
>         __copy_move_b(const _Tp* __first, const _Tp* __last, _Tp* __resul=
t)
>         {
> -#if __cplusplus >=3D 201103L
> -         using __assignable =3D __conditional_t<_IsMove,
> -                                              is_move_assignable<_Tp>,
> -                                              is_copy_assignable<_Tp>>;
> -         // trivial types can have deleted assignment
> -         static_assert( __assignable::value, "type must be assignable" );
> -#endif
>           const ptrdiff_t _Num =3D __last - __first;
> -         if (_Num)
> +         if (__builtin_expect(_Num > 1, true))
>             __builtin_memmove(__result - _Num, __first, sizeof(_Tp) * _Nu=
m);
> +         else if (_Num =3D=3D 1)
> +           *__result =3D *__first;
>           return __result - _Num;
>         }
>      };


If this code path also takes care of std::move(b,e,o), then this doesn't so=
und
correct -- for _Num=3D=3D1 and _IsMove=3D=3Dtrue, then it should use a move=
 assignment
(std::move(*__first)). But then that std::move doesn't actually work as __f=
irst
is a pointer to const...=