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

[redi at gcc dot gnu.org via Gcc-bugs]

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

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(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;
        }
     };