[PATCH] libstdc++/77334 move assign RB trees of non-copyable types

[Jonathan Wakely]

On 27/08/16 21:29 +0200, François Dumont wrote:
>On 23/08/2016 15:17, Jonathan Wakely wrote:
>>This fixes move assignment of RB trees to tag dispatch so the branch
>>that copies nodes doesn't need to be well-formed.
>>
>>    PR libstdc++/77334
>>    * include/bits/stl_tree.h (_Rb_tree::_M_move_assign): New functions.
>>    (_Rb_tree::operator=(_Rb_tree&&)): Dispatch to _M_move_assign.
>>    * testsuite/23_containers/map/77334.cc: New test.
>>
>>Tested powerpc64le-linux, committed to trunk.
>>
>>Backports to 5 and 6 to follow.
>>
>+    operator=(_Rb_tree&& __x)
>+    noexcept(_Alloc_traits::_S_nothrow_move()
>+	     && is_nothrow_move_assignable<_Compare>::value)
>+    {
>+      _M_impl._M_key_compare = __x._M_impl._M_key_compare;
>
>
>Shouldn't it be:
>
>+      _M_impl._M_key_compare = std::move(__x._M_impl._M_key_compare);
>
>I can't find anything in Standard saying to do so but wouldn't it be more natural ?

Yes, I can't think of any reason that would cause a problem. The
moved-from container is empty after the move assignment so even if
moving the comparison object changes the order it defines, no elements
will be left in an invalid order.

We don't need to use the comparison object after it's moved, but I'll
add a test to guarantee that remains true.