[committed] libstdc++: Implement string_view range constructor for C++23

Tue Apr 27 14:34:01 GMT 2021

On 23/03/2021 00:16, Jonathan Wakely via Gcc-patches wrote:
> This implements the new string_view constructor proposed by P1989R2.
> This hasn't been voted into the C++23 draft yet, but it's been reviewed
> by LWG and is expected to be approved at the next WG21 meeting.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/std/string_view (basic_string_view(Range&&)): Define new
> 	constructor and deduction guide.
> 	* testsuite/21_strings/basic_string_view/cons/char/range_c++20.cc: New test.
> 	* testsuite/21_strings/basic_string_view/cons/wchar_t/range_c++20.cc: New test.
> 
> Tested powerpc64le-linux. Committed to trunk.
> 
> (I wrote "C++20" in the commit log summary but it's a C++23 feature,
> which is why I'm confortable pushing it now in stage 4.)

I observe that with this change, recent Clang 13 trunk with -std=c++2b 
starts to fail on Boost's boost/spirit/include/qi.hpp (at least both 
Boost 1.73 and 1.75).  I unfortunately wasn't able to reduce this to a 
reasonably small testcase, and have no idea whether its a Clang, 
libstdc++, or a even Boost issue.

> $ cat test.cc
> #include "boost/spirit/include/qi.hpp"

> $ ~/llvm/trunk/inst/bin/clang++ --gcc-toolchain=~/gcc/trunk/inst -std=c++2b -fsyntax-only test.cc
> In file included from test.cc:1:
> In file included from /usr/include/boost/spirit/include/qi.hpp:16:
> In file included from /usr/include/boost/spirit/home/qi.hpp:14:
> In file included from /usr/include/boost/spirit/home/qi/action.hpp:14:
> In file included from /usr/include/boost/spirit/home/qi/action/action.hpp:14:
> In file included from /usr/include/boost/spirit/home/qi/meta_compiler.hpp:14:
> In file included from /usr/include/boost/spirit/home/support/meta_compiler.hpp:17:
> In file included from /usr/include/boost/spirit/home/support/make_component.hpp:15:
> In file included from /usr/include/boost/spirit/home/support/detail/make_cons.hpp:16:
> In file included from /usr/include/boost/fusion/include/cons.hpp:10:
> In file included from /usr/include/boost/fusion/support/config.hpp:13:
> In file included from ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/utility:70:
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/stl_pair.h:217:11: error: field has incomplete type 'boost::spirit::info'
>       _T1 first;                 ///< The first member
>           ^
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/iterator_concepts.h:949:29: note: in instantiation of template class 'std::pair<boost::spirit::info, boost::spirit::info>' requested here
>           { __detail::__decay_copy(begin(__t)) } -> input_or_output_iterator;
>                                    ^
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/iterator_concepts.h:949:6: note: in instantiation of requirement here
>           { __detail::__decay_copy(begin(__t)) } -> input_or_output_iterator;
>             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/iterator_concepts.h:947:5: note: while substituting template arguments into constraint expression here
>         && requires(_Tp& __t)
>            ^~~~~~~~~~~~~~~~~~
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/ranges_base.h:115:7: note: while checking the satisfaction of concept '__adl_begin<const boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>> &>' requested here
>           || __adl_begin<_Tp>
>              ^~~~~~~~~~~~~~~~
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/ranges_base.h:115:7: note: while substituting template arguments into constraint expression here
>           || __adl_begin<_Tp>
>              ^~~~~~~~~~~~~~~~
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/ranges_base.h:581:2: note: (skipping 83 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
>         ranges::begin(__t);
>         ^
> /usr/include/boost/mpl/aux_/preprocessed/gcc/and.hpp:50:7: note: in instantiation of template class 'boost::mpl::aux::and_impl<true, boost::mpl::not_<boost::is_const<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>>>>, boost::detail::variant::is_variant_constructible_from<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&, boost::mpl::l_item<mpl_::long_<5>, boost::spirit::info::nil_, boost::mpl::l_item<mpl_::long_<4>, std::basic_string<char>, boost::mpl::l_item<mpl_::long_<3>, boost::recursive_wrapper<boost::spirit::info>, boost::mpl::l_item<mpl_::long_<2>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::mpl::l_item<mpl_::long_<1>, boost::recursive_wrapper<std::list<boost::spirit::info>>, boost::mpl::l_end>>>>>>, mpl_::bool_<true>, mpl_::bool_<true>>' requested here
>     : aux::and_impl<
>       ^
> /usr/include/boost/core/enable_if.hpp:41:41: note: in instantiation of template class 'boost::mpl::and_<boost::is_rvalue_reference<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&>, boost::mpl::not_<boost::is_const<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>>>>, boost::detail::variant::is_variant_constructible_from<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&, boost::mpl::l_item<mpl_::long_<5>, boost::spirit::info::nil_, boost::mpl::l_item<mpl_::long_<4>, std::basic_string<char>, boost::mpl::l_item<mpl_::long_<3>, boost::recursive_wrapper<boost::spirit::info>, boost::mpl::l_item<mpl_::long_<2>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::mpl::l_item<mpl_::long_<1>, boost::recursive_wrapper<std::list<boost::spirit::info>>, boost::mpl::l_end>>>>>>>' requested here
>   struct enable_if : public enable_if_c<Cond::value, T> {};
>                                         ^
> /usr/include/boost/variant/variant.hpp:2136:21: note: in instantiation of template class 'boost::enable_if<boost::mpl::and_<boost::is_rvalue_reference<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&>, boost::mpl::not_<boost::is_const<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>>>>, boost::detail::variant::is_variant_constructible_from<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&, boost::mpl::l_item<mpl_::long_<5>, boost::spirit::info::nil_, boost::mpl::l_item<mpl_::long_<4>, std::basic_string<char>, boost::mpl::l_item<mpl_::long_<3>, boost::recursive_wrapper<boost::spirit::info>, boost::mpl::l_item<mpl_::long_<2>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::mpl::l_item<mpl_::long_<1>, boost::recursive_wrapper<std::list<boost::spirit::info>>, boost::mpl::l_end>>>>>>>, boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &>' requested here
>     typename boost::enable_if<
>                     ^
> /usr/include/boost/variant/variant.hpp:1105:7: note: while substituting deduced template arguments into function template 'operator=' [with T = boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>>]
> class variant
>       ^
> /usr/include/boost/spirit/home/support/info.hpp:29:12: note: while declaring the implicit move assignment operator for 'info'
>     struct info
>            ^
> /usr/include/boost/spirit/home/support/info.hpp:29:12: note: definition of 'boost::spirit::info' is not complete until the closing '}'
> In file included from test.cc:1:
> In file included from /usr/include/boost/spirit/include/qi.hpp:16:
> In file included from /usr/include/boost/spirit/home/qi.hpp:14:
> In file included from /usr/include/boost/spirit/home/qi/action.hpp:14:
> In file included from /usr/include/boost/spirit/home/qi/action/action.hpp:14:
> In file included from /usr/include/boost/spirit/home/qi/meta_compiler.hpp:14:
> In file included from /usr/include/boost/spirit/home/support/meta_compiler.hpp:17:
> In file included from /usr/include/boost/spirit/home/support/make_component.hpp:15:
> In file included from /usr/include/boost/spirit/home/support/detail/make_cons.hpp:16:
> In file included from /usr/include/boost/fusion/include/cons.hpp:10:
> In file included from /usr/include/boost/fusion/support/config.hpp:13:
> In file included from ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/utility:70:
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/stl_pair.h:218:11: error: field has incomplete type 'boost::spirit::info'
>       _T2 second;                ///< The second member
>           ^
> /usr/include/boost/spirit/home/support/info.hpp:29:12: note: definition of 'boost::spirit::info' is not complete until the closing '}'
>     struct info
>            ^
> In file included from test.cc:1:
> In file included from /usr/include/boost/spirit/include/qi.hpp:16:
> In file included from /usr/include/boost/spirit/home/qi.hpp:14:
> In file included from /usr/include/boost/spirit/home/qi/action.hpp:14:
> In file included from /usr/include/boost/spirit/home/qi/action/action.hpp:14:
> In file included from /usr/include/boost/spirit/home/qi/meta_compiler.hpp:14:
> In file included from /usr/include/boost/spirit/home/support/meta_compiler.hpp:17:
> In file included from /usr/include/boost/spirit/home/support/make_component.hpp:15:
> In file included from /usr/include/boost/spirit/home/support/detail/make_cons.hpp:16:
> In file included from /usr/include/boost/fusion/include/cons.hpp:10:
> In file included from /usr/include/boost/fusion/support/config.hpp:13:
> In file included from ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/utility:70:
> In file included from ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/stl_pair.h:59:
> In file included from ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/move.h:57:
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/type_traits:1060:7: error: static_assert failed due to requirement 'std::__is_complete_or_unbounded(std::__type_identity<boost::spirit::info>{})' "template argument must be a complete class or an unbounded array"
>       static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
>       ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/type_traits:139:26: note: in instantiation of template class 'std::is_copy_assignable<boost::spirit::info>' requested here
>     : public conditional<_B1::value, _B2, _B1>::type
>                          ^
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/stl_pair.h:391:3: note: in instantiation of template class 'std::__and_<std::is_copy_assignable<boost::spirit::info>, std::is_copy_assignable<boost::spirit::info>>' requested here
>                 __and_<is_copy_assignable<_T1>,
>                 ^
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/iterator_concepts.h:949:29: note: in instantiation of template class 'std::pair<boost::spirit::info, boost::spirit::info>' requested here
>           { __detail::__decay_copy(begin(__t)) } -> input_or_output_iterator;
>                                    ^
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/iterator_concepts.h:949:6: note: in instantiation of requirement here
>           { __detail::__decay_copy(begin(__t)) } -> input_or_output_iterator;
>             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/iterator_concepts.h:947:5: note: while substituting template arguments into constraint expression here
>         && requires(_Tp& __t)
>            ^~~~~~~~~~~~~~~~~~
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/ranges_base.h:115:7: note: (skipping 85 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
>           || __adl_begin<_Tp>
>              ^
> /usr/include/boost/mpl/aux_/preprocessed/gcc/and.hpp:50:7: note: in instantiation of template class 'boost::mpl::aux::and_impl<true, boost::mpl::not_<boost::is_const<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>>>>, boost::detail::variant::is_variant_constructible_from<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&, boost::mpl::l_item<mpl_::long_<5>, boost::spirit::info::nil_, boost::mpl::l_item<mpl_::long_<4>, std::basic_string<char>, boost::mpl::l_item<mpl_::long_<3>, boost::recursive_wrapper<boost::spirit::info>, boost::mpl::l_item<mpl_::long_<2>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::mpl::l_item<mpl_::long_<1>, boost::recursive_wrapper<std::list<boost::spirit::info>>, boost::mpl::l_end>>>>>>, mpl_::bool_<true>, mpl_::bool_<true>>' requested here
>     : aux::and_impl<
>       ^
> /usr/include/boost/core/enable_if.hpp:41:41: note: in instantiation of template class 'boost::mpl::and_<boost::is_rvalue_reference<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&>, boost::mpl::not_<boost::is_const<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>>>>, boost::detail::variant::is_variant_constructible_from<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&, boost::mpl::l_item<mpl_::long_<5>, boost::spirit::info::nil_, boost::mpl::l_item<mpl_::long_<4>, std::basic_string<char>, boost::mpl::l_item<mpl_::long_<3>, boost::recursive_wrapper<boost::spirit::info>, boost::mpl::l_item<mpl_::long_<2>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::mpl::l_item<mpl_::long_<1>, boost::recursive_wrapper<std::list<boost::spirit::info>>, boost::mpl::l_end>>>>>>>' requested here
>   struct enable_if : public enable_if_c<Cond::value, T> {};
>                                         ^
> /usr/include/boost/variant/variant.hpp:2136:21: note: in instantiation of template class 'boost::enable_if<boost::mpl::and_<boost::is_rvalue_reference<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&>, boost::mpl::not_<boost::is_const<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>>>>, boost::detail::variant::is_variant_constructible_from<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&, boost::mpl::l_item<mpl_::long_<5>, boost::spirit::info::nil_, boost::mpl::l_item<mpl_::long_<4>, std::basic_string<char>, boost::mpl::l_item<mpl_::long_<3>, boost::recursive_wrapper<boost::spirit::info>, boost::mpl::l_item<mpl_::long_<2>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::mpl::l_item<mpl_::long_<1>, boost::recursive_wrapper<std::list<boost::spirit::info>>, boost::mpl::l_end>>>>>>>, boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &>' requested here
>     typename boost::enable_if<
>                     ^
> /usr/include/boost/variant/variant.hpp:1105:7: note: while substituting deduced template arguments into function template 'operator=' [with T = boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>>]
> class variant
>       ^
> /usr/include/boost/spirit/home/support/info.hpp:29:12: note: while declaring the implicit move assignment operator for 'info'
>     struct info
>            ^
> In file included from test.cc:1:
> In file included from /usr/include/boost/spirit/include/qi.hpp:16:
> In file included from /usr/include/boost/spirit/home/qi.hpp:14:
> In file included from /usr/include/boost/spirit/home/qi/action.hpp:14:
> In file included from /usr/include/boost/spirit/home/qi/action/action.hpp:14:
> In file included from /usr/include/boost/spirit/home/qi/meta_compiler.hpp:14:
> In file included from /usr/include/boost/spirit/home/support/meta_compiler.hpp:17:
> In file included from /usr/include/boost/spirit/home/support/make_component.hpp:15:
> In file included from /usr/include/boost/spirit/home/support/detail/make_cons.hpp:16:
> In file included from /usr/include/boost/fusion/include/cons.hpp:10:
> In file included from /usr/include/boost/fusion/support/config.hpp:13:
> In file included from ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/utility:70:
> In file included from ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/stl_pair.h:59:
> In file included from ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/move.h:57:
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/type_traits:1081:7: error: static_assert failed due to requirement 'std::__is_complete_or_unbounded(std::__type_identity<boost::spirit::info>{})' "template argument must be a complete class or an unbounded array"
>       static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
>       ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/type_traits:139:26: note: in instantiation of template class 'std::is_move_assignable<boost::spirit::info>' requested here
>     : public conditional<_B1::value, _B2, _B1>::type
>                          ^
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/stl_pair.h:402:3: note: in instantiation of template class 'std::__and_<std::is_move_assignable<boost::spirit::info>, std::is_move_assignable<boost::spirit::info>>' requested here
>                 __and_<is_move_assignable<_T1>,
>                 ^
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/iterator_concepts.h:949:29: note: in instantiation of template class 'std::pair<boost::spirit::info, boost::spirit::info>' requested here
>           { __detail::__decay_copy(begin(__t)) } -> input_or_output_iterator;
>                                    ^
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/iterator_concepts.h:949:6: note: in instantiation of requirement here
>           { __detail::__decay_copy(begin(__t)) } -> input_or_output_iterator;
>             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/iterator_concepts.h:947:5: note: while substituting template arguments into constraint expression here
>         && requires(_Tp& __t)
>            ^~~~~~~~~~~~~~~~~~
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/ranges_base.h:115:7: note: (skipping 85 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
>           || __adl_begin<_Tp>
>              ^
> /usr/include/boost/mpl/aux_/preprocessed/gcc/and.hpp:50:7: note: in instantiation of template class 'boost::mpl::aux::and_impl<true, boost::mpl::not_<boost::is_const<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>>>>, boost::detail::variant::is_variant_constructible_from<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&, boost::mpl::l_item<mpl_::long_<5>, boost::spirit::info::nil_, boost::mpl::l_item<mpl_::long_<4>, std::basic_string<char>, boost::mpl::l_item<mpl_::long_<3>, boost::recursive_wrapper<boost::spirit::info>, boost::mpl::l_item<mpl_::long_<2>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::mpl::l_item<mpl_::long_<1>, boost::recursive_wrapper<std::list<boost::spirit::info>>, boost::mpl::l_end>>>>>>, mpl_::bool_<true>, mpl_::bool_<true>>' requested here
>     : aux::and_impl<
>       ^
> /usr/include/boost/core/enable_if.hpp:41:41: note: in instantiation of template class 'boost::mpl::and_<boost::is_rvalue_reference<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&>, boost::mpl::not_<boost::is_const<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>>>>, boost::detail::variant::is_variant_constructible_from<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&, boost::mpl::l_item<mpl_::long_<5>, boost::spirit::info::nil_, boost::mpl::l_item<mpl_::long_<4>, std::basic_string<char>, boost::mpl::l_item<mpl_::long_<3>, boost::recursive_wrapper<boost::spirit::info>, boost::mpl::l_item<mpl_::long_<2>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::mpl::l_item<mpl_::long_<1>, boost::recursive_wrapper<std::list<boost::spirit::info>>, boost::mpl::l_end>>>>>>>' requested here
>   struct enable_if : public enable_if_c<Cond::value, T> {};
>                                         ^
> /usr/include/boost/variant/variant.hpp:2136:21: note: in instantiation of template class 'boost::enable_if<boost::mpl::and_<boost::is_rvalue_reference<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&>, boost::mpl::not_<boost::is_const<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>>>>, boost::detail::variant::is_variant_constructible_from<boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &&, boost::mpl::l_item<mpl_::long_<5>, boost::spirit::info::nil_, boost::mpl::l_item<mpl_::long_<4>, std::basic_string<char>, boost::mpl::l_item<mpl_::long_<3>, boost::recursive_wrapper<boost::spirit::info>, boost::mpl::l_item<mpl_::long_<2>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::mpl::l_item<mpl_::long_<1>, boost::recursive_wrapper<std::list<boost::spirit::info>>, boost::mpl::l_end>>>>>>>, boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>> &>' requested here
>     typename boost::enable_if<
>                     ^
> /usr/include/boost/variant/variant.hpp:1105:7: note: while substituting deduced template arguments into function template 'operator=' [with T = boost::variant<boost::spirit::info::nil_, std::basic_string<char>, boost::recursive_wrapper<boost::spirit::info>, boost::recursive_wrapper<std::pair<boost::spirit::info, boost::spirit::info>>, boost::recursive_wrapper<std::list<boost::spirit::info>>>]
> class variant
>       ^
> /usr/include/boost/spirit/home/support/info.hpp:29:12: note: while declaring the implicit move assignment operator for 'info'
>     struct info
>            ^
> 4 errors generated.