[PATCH] PR libstdc++/86127 avoid unnecessary allocator conversions

[François Dumont]

You're still using __gnu_cxx::__alloc_traits for 
_Fwd_list_base::_Node_alloc_traits to get the _S_always_equal().

So you might perhaps cleanup this part of the ChangeLog entry.

Or make this true by making noexcept qualification dependent on the 
underlying constructors like I did for rb_tree:

 Â Â Â Â Â  _Rb_tree(_Rb_tree&& __x, _Node_allocator&& __a)
 Â Â Â Â Â  noexcept( noexcept(
 Â Â Â  _Rb_tree(std::declval<_Rb_tree&&>(), std::declval<_Node_allocator&&>(),
 Â Â Â  Â Â Â  Â std::declval<typename _Alloc_traits::is_always_equal>())) )


On 13/06/2018 17:13, Jonathan Wakely wrote:
> There is no need to use an allocator of the correct value_type when
> calling allocator_traits::construct and allocator_traits::destroy. The
> existing node allocator can be used, instead of constructing a new
> allocator object every time.
>
> There's also no benefit to using __gnu_cxx::__alloc_traits instead of
> std::allocator_traits to get the pointer and const_pointer types.
> std::forward_list is only available for C++11 and later, when
> std::allocator_traits is available too.
>
> Â Â Â Â PR libstdc++/86127
> Â Â Â Â * include/bits/forward_list.h (_Fwd_list_base::_Tp_alloc_type): 
> Remove
> Â Â Â Â unused typedef.
> Â Â Â Â (_Fwd_list_base::_Node_alloc_traits): Use allocator_traits instead of
> Â Â Â Â __gnu_cxx::__alloc_traits.
> Â Â Â Â (_Fwd_list_base::_M_create_node, _Fwd_list_base::_M_erase_after):
> Â Â Â Â Use node allocator to create and destroy elements.
> Â Â Â Â (forward_list::_Tp_alloc_type): Remove unused typedef.
> Â Â Â Â (forward_list::_Alloc_traits): Use allocator_traits instead of
> Â Â Â Â __gnu_cxx::__alloc_traits.
>
> Tested powerpc64le-linux, committed to trunk.
>
>