[Bug libstdc++/108846] std::copy, std::copy_n and std::copy_backward on potentially overlapping subobjects

["dangelog at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108846

--- 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
> > 
> > Is the extent of the fix just to add another branch to the if (_Num) check
> > here?
> 
> Yes, I think that's the simplest place to fix it.
> 
> And we can even get rid of the static assertion, because the Num==1 branch
> will require assignability now.
> 
> --- 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* __result)
>         {
> -#if __cplusplus >= 201103L
> -         using __assignable = __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 = __last - __first;
> -         if (_Num)
> +         if (__builtin_expect(_Num > 1, true))
>             __builtin_memmove(__result - _Num, __first, sizeof(_Tp) * _Num);
> +         else if (_Num == 1)
> +           *__result = *__first;
>           return __result - _Num;
>         }
>      };


If this code path also takes care of std::move(b,e,o), then this doesn't sound
correct -- for _Num==1 and _IsMove==true, then it should use a move assignment
(std::move(*__first)). But then that std::move doesn't actually work as __first
is a pointer to const...