Hi

I've eventually completed this work.

This evolution will allow to build libstdc++ without dual abi and using 
cxx11 abi. For the moment such a config is only accessible through the 
--enable-symvers=gnu-versioned-namespace configuration.

     libstdc++: [_GLIBCXX_INLINE_VERSION] Use cxx11 abi [PR83077]

     Use cxx11 abi when activating versioned namespace mode.

     libstdcxx-v3/ChangeLog:

             PR libstdc++/83077
             * acinclude.m4 [GLIBCXX_ENABLE_LIBSTDCXX_DUAL_ABI]: Default 
to "new" libstdcxx abi.
             * config/locale/dragonfly/monetary_members.cc 
[!_GLIBCXX_USE_DUAL_ABI]: Define money_base
             members.
             * config/locale/generic/monetary_members.cc 
[!_GLIBCXX_USE_DUAL_ABI]: Likewise.
             * config/locale/gnu/monetary_members.cc 
[!_GLIBCXX_USE_DUAL_ABI]: Likewise.
             * config/locale/gnu/numeric_members.cc
             [!_GLIBCXX_USE_DUAL_ABI](__narrow_multibyte_chars): Define.
             * configure: Regenerate.
             * include/bits/c++config
             [_GLIBCXX_INLINE_VERSION](_GLIBCXX_NAMESPACE_CXX11, 
_GLIBCXX_BEGIN_NAMESPACE_CXX11):
             Define empty.
[_GLIBCXX_INLINE_VERSION](_GLIBCXX_END_NAMESPACE_CXX11, 
_GLIBCXX_DEFAULT_ABI_TAG):
             Likewise.
             * include/bits/cow_string.h [!_GLIBCXX_USE_CXX11_ABI]: 
Define a light version of COW
             basic_string as __std_cow_string for use in stdexcept.
             * include/std/stdexcept [_GLIBCXX_USE_CXX11_ABI]: Define 
__cow_string.
             (__cow_string(const char*)): New.
             (__cow_string::c_str()): New.
             * python/libstdcxx/v6/printers.py 
(StdStringPrinter::__init__): Set self.new_string to True
             when std::__8::basic_string type is found.
             * src/Makefile.am 
[ENABLE_SYMVERS_GNU_NAMESPACE](ldbl_alt128_compat_sources): Define empty.
             * src/Makefile.in: Regenerate.
             * src/c++11/Makefile.am (cxx11_abi_sources): Rename into...
             (dual_abi_sources): ...this. Also move cow-local_init.cc, 
cxx11-hash_tr1.cc,
             cxx11-ios_failure.cc entries to...
             (sources): ...this.
             (extra_string_inst_sources): Move cow-fstream-inst.cc, 
cow-sstream-inst.cc, cow-string-inst.cc,
             cow-string-io-inst.cc, cow-wtring-inst.cc, 
cow-wstring-io-inst.cc, cxx11-locale-inst.cc,
             cxx11-wlocale-inst.cc entries to...
             (inst_sources): ...this.
             * src/c++11/Makefile.in: Regenerate.
             * src/c++11/cow-fstream-inst.cc [_GLIBCXX_USE_CXX11_ABI]: 
Skip definitions.
             * src/c++11/cow-locale_init.cc [_GLIBCXX_USE_CXX11_ABI]: 
Skip definitions.
             * src/c++11/cow-sstream-inst.cc [_GLIBCXX_USE_CXX11_ABI]: 
Skip definitions.
             * src/c++11/cow-stdexcept.cc [_GLIBCXX_USE_CXX11_ABI]: 
Include <bits/cow_string.h>.
             [_GLIBCXX_USE_DUAL_ABI || 
_GLIBCXX_USE_CXX11_ABI](__cow_string): Redefine before including
             <stdexcept>. Define 
_GLIBCXX_DEFINE_STDEXCEPT_INSTANTIATIONS so that __cow_string definition
             in <stdexcept> is skipped.
             [_GLIBCXX_USE_CXX11_ABI]: Skip Transaction Memory TS 
definitions.
             * src/c++11/cow-string-inst.cc [_GLIBCXX_USE_CXX11_ABI]: 
Skip definitions.
             * src/c++11/cow-string-io-inst.cc [_GLIBCXX_USE_CXX11_ABI]: 
Skip definitions.
             * src/c++11/cow-wstring-inst.cc [_GLIBCXX_USE_CXX11_ABI]: 
Skip definitions.
             * src/c++11/cow-wstring-io-inst.cc 
[_GLIBCXX_USE_CXX11_ABI]: Skip definitions.
             * src/c++11/cxx11-hash_tr1.cc [!_GLIBCXX_USE_CXX11_ABI]: 
Skip definitions.
             * src/c++11/cxx11-ios_failure.cc [!_GLIBCXX_USE_CXX11_ABI]: 
Skip definitions.
             [!_GLIBCXX_USE_DUAL_ABI] (__ios_failure): Remove.
             * src/c++11/cxx11-locale-inst.cc: Cleanup, just include 
locale-inst.cc.
             * src/c++11/cxx11-stdexcept.cc [!_GLIBCXX_USE_CXX11_ABI]: 
Skip definitions.
             * src/c++11/cxx11-wlocale-inst.cc 
[!_GLIBCXX_USE_CXX11_ABI]: Skip definitions.
             * src/c++11/locale-inst-numeric.h
[!_GLIBCXX_USE_DUAL_ABI](std::use_facet<num_get<>>, 
std::use_facet<num_put<>>): Instantiate.
[!_GLIBCXX_USE_DUAL_ABI](std::has_facet<num_get<>>, 
std::has_facet<num_put<>>): Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](std::num_get<C, 
istreambuf_iterator<C>>): Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](std::num_put<C, 
ostreambuf_iterator<C>>): Instantiate.
             * src/c++11/locale-inst.cc [!_GLIBCXX_USE_DUAL_ABI]: Build 
only when configured
             _GLIBCXX_USE_CXX11_ABI is equal to currently built abi.
             [!_GLIBCXX_USE_DUAL_ABI](__moneypunct_cache<C, false>): 
Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](__moneypunct_cache<C, true>): 
Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](__numpunct_cache<C>): Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](__timepunct<C>): Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](__timepunct_cache<C>): Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](time_put<C, 
ostreambuf_iterator<C>>): Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](time_put_byname<C, 
ostreambuf_iterator<C>>): Instantiate.
[!_GLIBCXX_USE_DUAL_ABI](__ctype_abstract_base<C>): Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](ctype_byname<C>): Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](__codecvt_abstract_base<C, char, 
mbstate_t>): Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](codecvt_byname<C, char, 
mbstate_t>): Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](codecvt_byname<C, char, 
mbstate_t>): Instantiate.
[!_GLIBCXX_USE_DUAL_ABI](use_facet<ctype<C>>(const locale&)): Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](use_facet<codecvt<C, char, 
mbstate_t>>(const locale&)): Instantiate.
[!_GLIBCXX_USE_DUAL_ABI](use_facet<__timepunct<C>>(const locale&)): 
Instantiate.
[!_GLIBCXX_USE_DUAL_ABI](use_facet<time_put<C>>(const locale&)): 
Instantiate.
[!_GLIBCXX_USE_DUAL_ABI](has_facet<ctype<C>>(const locale&)): Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](has_facet<codecvt<C, char, 
mbstate_t>>(const locale&)): Instantiate.
[!_GLIBCXX_USE_DUAL_ABI](has_facet<__timepunct<C>>(const locale&)): 
Instantiate.
[!_GLIBCXX_USE_DUAL_ABI](has_facet<time_put<C>>(const locale&)): 
Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](__add_grouping<C>): Define.
             [!_GLIBCXX_USE_DUAL_ABI](__pad<C, char_traits<C>>): 
Instantiate.
             [!_GLIBCXX_USE_DUAL_ABI](__int_to_char(C*, unsigned long, 
const C*,
             ios_base::fmtflags, bool)): Define.
             [!_GLIBCXX_USE_DUAL_ABI](__int_to_char(C*, unsigned long 
long, const C*,
             ios_base::fmtflags, bool)): Define.
             * src/c++11/string-inst.cc 
(_GLIBCXX_DEFINING_CXX11_ABI_INSTANTIATIONS): Define following
             _GLIBCXX_USE_CXX11_ABI value.
             [_GLIBCXX_USE_CXX11_ABI && 
!_GLIBCXX_DEFINING_CXX11_ABI_INSTANTIATIONS]:
             Define _GLIBCXX_DEFINING_COW_STRING_INSTANTIATIONS. Include 
<bits/cow_string.h>.
             Define basic_string as __std_cow_string for the current 
translation unit.
             * src/c++11/wlocale-inst.cc [!_GLIBCXX_USE_CXX11_ABI]: Skip 
definitions.
             * src/c++98/Makefile.am (cxx11_abi_sources): Remove, unique 
cow-istream-string.cc entry
             move to...
             (inst_sources): ...this.
             * src/c++98/Makefile.in: Regenerate.
             * src/c++98/cow-istream-string.cc: Include <bits/c++config.h>.
             [_GLIBCXX_USE_CXX11_ABI]: Skip definitions.
             * src/c++98/hash_tr1.cc [_GLIBCXX_USE_CXX11_ABI]: Skip 
definitions.
             * src/c++98/ios_failure.cc 
[_GLIBCXX_USE_CXX11_ABI][_GLIBCXX_USE_DUAL_ABI]: Skip definitions.
             * src/c++98/istream-string.cc [!_GLIBCXX_USE_DUAL_ABI]: 
Build only when configured
             _GLIBCXX_USE_CXX11_ABI is equal to currently built abi.
             * src/c++98/locale_facets.cc 
[_GLIBCXX_USE_CXX11_ABI](__verify_grouping): Remove.
             * src/c++98/stdexcept.cc
             [_GLIBCXX_USE_CXX11_ABI](logic_error(const string&): Remove.
             [_GLIBCXX_USE_CXX11_ABI](domain_error(const string&): Remove.
             [_GLIBCXX_USE_CXX11_ABI](invalid_argument(const string&): 
Remove.
             [_GLIBCXX_USE_CXX11_ABI](length_error(const string&): Remove.
             [_GLIBCXX_USE_CXX11_ABI](out_of_range(const string&): Remove.
             [_GLIBCXX_USE_CXX11_ABI](runtime_error(const string&): Remove.
             [_GLIBCXX_USE_CXX11_ABI](range_error(const string&): Remove.
             [_GLIBCXX_USE_CXX11_ABI](overflow_error(const string&): Remove.
             [_GLIBCXX_USE_CXX11_ABI](underflow_error(const string&): 
Remove.

Tested under linux x86_64 with following configs:

--enable-symvers=gnu-versioned-namespace

--disable-libstdcxx-dual-abi

Ok to commit ?

François