[patch] Use abi_tag attribute on std::list

[Marc Glisse]

On Fri, 3 Oct 2014, Jonathan Wakely wrote:

> This is the patch I intend to commit to make std::list::size() O(1) as
> required by C++11.
>
> This is an ABI change, so std::list will get tagged with
> abi_tag("cxx11") so that it mangles differently.

Assuming a future where we have both _GLIBCXX_ABI_TAG_CXX11 and 
_GLIBCXX_ABI_TAG_CXX17, I don't really see how _GLIBCXX_DEFAULT_ABI_TAG
is supposed to work. We don't want to define _GLIBCXX_DEFAULT_ABI_TAG to 
_GLIBCXX_ABI_TAG_CXX17 and suddenly have std::list change mangling. Should 
it be called _GLIBCXX_DEFAULT_ABI_TAG_CXX11, meaning 
_GLIBCXX_ABI_TAG_CXX11_IF_ENABLED_AND_NOTHING_OTHERWISE?

Defining a dummy _M_distance in the old abi case is a bit strange (we 
could protect the single use with #if _GLIBCXX_USE_CXX11_ABI), but why 
not...

Do you mind if I move (in a future patch once yours is committed) _M_size 
into _M_impl::_M_node as suggested in PR 61347?

-- 
Marc Glisse