Here is another version with enhanced sizeof/alignof static_assert in 
string-inst.cc for the std::__cow_string definition from <stdexcept>. 
The assertions in cow-stdexcept.cc are now checking the definition which 
is in the same file.

On 13/08/2023 15:27, François Dumont wrote:
>
> Here is the fixed patch tested in all 3 modes:
>
> - _GLIBCXX_USE_DUAL_ABI
>
> - !_GLIBCXX_USE_DUAL_ABI && !_GLIBCXX_USE_CXX11_ABI
>
> - !_GLIBCXX_USE_DUAL_ABI && _GLIBCXX_USE_CXX11_ABI
>
> I don't know what you have in mind for the change below but I wanted 
> to let you know that I tried to put COW std::basic_string into a 
> nested __cow namespace when _GLIBCXX_USE_CXX11_ABI. But it had more 
> impact on string-inst.cc so I preferred the macro substitution approach.
>
> There are some test failing when !_GLIBCXX_USE_CXX11_ABI that are 
> unrelated with my changes. I'll propose fixes in coming days.
>
>     libstdc++: [_GLIBCXX_INLINE_VERSION] Use cxx11 abi [PR83077]
>
>     Use cxx11 abi when activating versioned namespace mode. To do support
>     a new configuration mode where !_GLIBCXX_USE_DUAL_ABI and 
> _GLIBCXX_USE_CXX11_ABI.
>
>     The main change is that std::__cow_string is now defined whenever 
> _GLIBCXX_USE_DUAL_ABI
>     or _GLIBCXX_USE_CXX11_ABI is true. Implementation is using 
> available std::string in
>     case of dual abi and a subset of it when it's not.
>
>     On the other side std::__sso_string is defined only when 
> _GLIBCXX_USE_DUAL_ABI is true
>     and _GLIBCXX_USE_CXX11_ABI is false. Meaning that 
> std::__sso_string is a typedef for the
>     cow std::string implementation when dual abi is disabled and cow 
> string is being used.
>
>     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.
>             Move static_assert to check std::_cow_string abi layout to...
>             * src/c++11/string-inst.cc: ...here.
>             (_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/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](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/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.
>
> Ok to commit ?
>
> François
>
> On 11/08/2023 09:51, Jonathan Wakely wrote:
>>
>>
>> On Fri, 11 Aug 2023, 06:44 François Dumont via Libstdc++, 
>> <libstdc++@gcc.gnu.org <mailto:libstdc%2B%2B@gcc.gnu.org>> wrote:
>>
>>     I hadn't tested the most basic default configuration and it is
>>     failing,
>>
>>
>> I did wonder about that when you said which configurations you had 
>> tested :)
>>
>>
>>
>>
>>     I need some more time yet.
>>
>>
>> OK, no problem.
>>
>> I actually have an idea for replacing the __cow_string hack with 
>> something better, which I will try to work on next week. That might 
>> make things simpler for you, as you won't need the __std_cow_string 
>> macro.
>>
>>
>>
>>
>>
>>     François
>>
>>
>>     On 10/08/2023 07:13, François Dumont wrote:
>>     > 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
>>     >
>>     >
>>