Status of std::string ABI change

["Peter A. Bigot" <pab at pabigot dot com>]

TL;DR: When will the underlying implementation of std::string in GCC
provde a non-copy-on-write implementation by default, through a
supported configuration-time option, or by selecting alternative
header/library configurations during application build?

Background: It's been known for a long time that libstdc++ std::string's
copy-on-write behavior causes problems.  It's also now non-conformant
under --std=c++11 (see
e.g. http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21334#c45).

<ext/vstring.h> provides __gnu_cxx::__vstring which behaves correctly,
but using it in current form appears to require source changes to the
application.  gcc 4.9.0 trunk as of 2013-11-24 still uses the
non-conforming std::string implementation for applications built with
--std=c++11.

Other libstdc++ bugs like 58265 are in suspended status with comments
like "until we break the ABI basic_string will remain nonconforming in
many serious ways."  I understand that "break the ABI" refers to the
consequences of switching out the string implementation and that this
has been delayed because of its significant impact on existing systems.

I've searched through bug reports, the 4.9 Changes page, the libstdc++
ISO status page in the manual, mailing list archives, Google, etc. and
have been unable to find an answer to the question above.

Thanks.

Peter