Document input_iterator incompatibility with pre-C++20 interfaces (was: Compilation failure on 1.83.0 release candidate (regression from 1.82.0))

[mclow]

Reported on the mailing list:

#include "boost/stl_interfaces/iterator_interface.hpp"
#include "boost/range/iterator_range.hpp"


struct basic_input_iter
   : boost::stl_interfaces::iterator_interface<
#if !BOOST_STL_INTERFACES_USE_DEDUCED_THIS
         basic_input_iter,
#endif
         std::input_iterator_tag, int>
{
   basic_input_iter() : it_(nullptr) {}
   basic_input_iter(int * it) : it_(it) {}

   int & operator*() const noexcept { return *it_; }
   basic_input_iter & operator++() noexcept
   {
       ++it_;
       return *this;
   }
   friend bool operator==(basic_input_iter lhs, basic_input_iter rhs) noexcept
   {
       return lhs.it_ == rhs.it_;
   }

   using base_type = boost::stl_interfaces::iterator_interface<
#if !BOOST_STL_INTERFACES_USE_DEDUCED_THIS
       basic_input_iter,
#endif
       std::input_iterator_tag, int>;
   using base_type::operator++;

private:
   int * it_;
};


auto make_range() {
 static int si;
 return boost::make_iterator_range(
     basic_input_iter(&si),
     basic_input_iter(&si + 1));
}

This fails to compile with the following errors:

g++ -Wall -Wstrict-aliasing=0 -Wno-unused-local-typedefs -Werror=return-type -static-libgcc -static-libasan -static-libstdc++ -static-libubsan -std=gnu++20 -O3 -flto -pthread -Iintermediate/linux64/obj/linux64__test__iter_test Iintermediate/linux64/libs/boost/install/include -DELDWOOD_EXTENSION_BOOST -c -o intermediate/linux64/obj/linux64__test__iter_test/eldwood/test/iter.cpp.o intermediate/linux64/obj/linux64__test__iter_test/eldwood/test/iter.cpp
In file included from intermediate/linux64/libs/boost/install/include/boost/iterator/iterator_facade.hpp:13,
from intermediate/linux64/libs/boost/install/include/boost/range/iterator_range_core.hpp:27,
from intermediate/linux64/libs/boost/install/include/boost/range/iterator_range.hpp:13,
from intermediate/linux64/obj/linux64__test__iter_test/eldwood/test/iter.cpp:2:
intermediate/linux64/libs/boost/install/include/boost/iterator/iterator_categories.hpp: In instantiation of ‘struct boost::iterators::iterator_traversal<basic_input_iter>’:
intermediate/linux64/libs/boost/install/include/boost/range/iterator_range_core.hpp:158:5: required from ‘struct boost::iterator_range_detail::pure_iterator_traversal<basic_input_iter>’
intermediate/linux64/libs/boost/install/include/boost/range/iterator_range_core.hpp:434:15: required from ‘class boost::iterator_range<basic_input_iter>’
intermediate/linux64/obj/linux64__test__iter_test/eldwood/test/iter.cpp:40:36: required from here
intermediate/linux64/libs/boost/install/include/boost/iterator/iterator_categories.hpp:118:8: error: no type named ‘iterator_category’ in ‘struct std::iterator_traits<basic_input_iter>’
118 | struct iterator_traversal
| ^~~~~~~~~~~~~~~~~~
intermediate/linux64/libs/boost/install/include/boost/range/iterator_range_core.hpp: In instantiation of ‘struct boost::iterator_range_detail::pure_iterator_traversal<basic_input_iter>’:
intermediate/linux64/libs/boost/install/include/boost/range/iterator_range_core.hpp:434:15: required from ‘class boost::iterator_range<basic_input_iter>’
intermediate/linux64/obj/linux64__test__iter_test/eldwood/test/iter.cpp:40:36: required from here
intermediate/linux64/libs/boost/install/include/boost/range/iterator_range_core.hpp:158:5: error: no type named ‘type’ in ‘struct boost::iterators::iterator_traversal<basic_input_iter>’
158 | traversal_t;
| ^~~~~~~~~~~
In file included from intermediate/linux64/libs/boost/install/include/boost/utility/enable_if.hpp:15,
from intermediate/linux64/libs/boost/install/include/boost/range/has_range_iterator.hpp:21,
from intermediate/linux64/libs/boost/install/include/boost/range/difference_type.hpp:21,
from intermediate/linux64/libs/boost/install/include/boost/range/size_type.hpp:19,
from intermediate/linux64/libs/boost/install/include/boost/range/size.hpp:21,
from intermediate/linux64/libs/boost/install/include/boost/range/functions.hpp:20,
from intermediate/linux64/libs/boost/install/include/boost/range/iterator_range_core.hpp:38:
intermediate/linux64/libs/boost/install/include/boost/core/enable_if.hpp: In instantiation of ‘struct boost::enable_if<boost::iterator_range<basic_input_iter>::is_compatible_range, void>’:
intermediate/linux64/libs/boost/install/include/boost/range/iterator_range_core.hpp:489:13: required by substitution of ‘template boost::iterator_range<basic_input_iter>::iterator_range(const SinglePassRange&, typename boost::enable_if<is_compatible_range, void>::type*) [with SinglePassRange = basic_input_iter]’
intermediate/linux64/libs/boost/install/include/boost/range/iterator_range_core.hpp:758:20: required from ‘boost::iterator_range boost::make_iterator_range(IteratorT, IteratorT) [with IteratorT = basic_input_iter]’
intermediate/linux64/obj/linux64__test__iter_test/eldwood/test/iter.cpp:40:36: required from here
intermediate/linux64/libs/boost/install/include/boost/core/enable_if.hpp:41:10: error: ‘value’ is not a member of ‘boost::iterator_range<basic_input_iter>::is_compatible_range’
41 | struct enable_if : public enable_if_c<Cond::value, T> {};
| ^~~~~~~~~
intermediate/linux64/libs/boost/install/include/boost/core/enable_if.hpp: In instantiation of ‘struct boost::enable_if<boost::iterator_range<basic_input_iter>::is_compatible_range<basic_input_iter>, void>’:
intermediate/linux64/libs/boost/install/include/boost/range/iterator_range_core.hpp:500:13: required by substitution of ‘template boost::iterator_range<basic_input_iter>::iterator_range(SinglePassRange&, typename boost::enable_if<is_compatible_range, void>::type*) [with SinglePassRange = basic_input_iter]’
intermediate/linux64/libs/boost/install/include/boost/range/iterator_range_core.hpp:758:20: required from ‘boost::iterator_range boost::make_iterator_range(IteratorT, IteratorT) [with IteratorT = basic_input_iter]’
intermediate/linux64/obj/linux64__test__iter_test/eldwood/test/iter.cpp:40:36: required from here
intermediate/linux64/libs/boost/install/include/boost/core/enable_if.hpp:41:10: error: ‘value’ is not a member of ‘boost::iterator_range<basic_input_iter>::is_compatible_range<basic_input_iter>’

Build failed.

[mclow]

Joaquín M López Muñoz wrote:

Looks like the iterator_concept -> iterator_category mapping used here is incomplete

The mapping is copied verbatim from zip_transform_view as spelled out at:

https://eel.is/c++draft/range.zip.transform.iterator#1

I guess this is ok for zip_transform_view, but the description of the mapping explicitly says

"The member typedef-name iterator​::​iterator_category is defined if and only if Base models forward_range."

Note the "if and only if", implying that if Base only models input_range then iterator​::​iterator_category
is not defined.

FWIW, the attached patch makes the snippet compile. It simply adds and input_iterator_tag -> input_iterator_tag
case to the mapping algorithm.

iterator_interface.hpp.patch

[mclow]

Looks like it came from this commit: 21e03e7

[mclow]

Zach says: Hey, Marshall. I'm on vacation and didn't even have my phone on yesterday. It looks like "Works as designed" to me. The design changed so that iterators are not compatible with pre 20 interfaces, based on LWG and LEWG feedback. The Boost library reflects that. I can take a closer look this weekend to verify.