Re: [boost] [range] concept checks break chained range adaptors

Subject: Re: [boost] [range] concept checks break chained range adaptors
From: Neil Groves (neil_at_[hidden])
Date: 2014-06-14 10:27:05

Hi!

On Sat, Jun 14, 2014 at 2:26 PM, Jürgen Hunold <jhunold_at_gmx.eu> wrote:

> Hi Neil,
>
> the fix for #9851 broke some of our code. We use chained range adaptors
> like
>
>
This fix simply enables BOOST_RANGE_CONCEPT_ASSERT. I had previously
disabled this checking on more compilers than I should have. This was
motivated to get code up and running on compilers that I didn't have access
to. I have since had users have a number of difficulties due that have been
difficult to diagnose due to having concept checking disabled. This is
particularly the case on later versions of Clang where the C++ support is
now excellent.

I therefore do not wish to revert the commit for reasons:
1. Any specific problem with particular compilers can be worked around by
simply disabling BOOST_RANGE_CONCEPT_ASSERT in boost/range/config.hpp
2. I will find ways of making concept assertions work by changing them in
the cases where there are problems
3. I will specifically disable concept assertions for problems for which I
fail to find solutions

> std::vector<std::string> separated
>  std::set<Foo> foos;
>
> boost::copy(separated |
>                 boost::adaptors::transformed(boost::bind(Foo::fromString,
> _1))
> |
>                 boost::adaptors::filtered(boost::bind(&Foo::isValid, _1)),
>                 std::inserter(foos, foos.end()));
>
> which fails with
>
>  error: constructor for 'boost::transform_iterator<boost::_bi::bind_t<Foo,
> Foo
> (*)(const std::__1::basic_string<char, std::__1::char_traits<char>,
> std::__1::allocator<char> > &), boost::_bi::list1<boost::arg<1> > >,
> std::__1::__wrap_iter<std::__1::basic_string<char,
> std::__1::char_traits<char>, std::__1::allocator<char> > *>,
> boost::use_default, boost::use_default>' must explicitly initialize the
> member
> 'm_f' which does not have a default constructor
>     transform_iterator() { }
>
> Full testcase and error message attached.
> This compiles and works with 1.55.0. git bisect shows that this fails to
> compile from
> 5f2560f - Ticket 9851 - adaptors should perform concept check assertions.
> for gcc and for clang starting with
> e43796c - allow clang to use Range Concepts despite reporting GCC 4.2.
> msvc-12.0 fails, too. gcc-4.8.2 and clang-3.5 (trunk) for the record.
>
>
I really appreciate the thorough report. I shall look at this problem over
the next couple of days and update the Trac ticket to report if I believe I
can get this fixed for the 1.56 release. My initial reading of this issue
is that the problem is with the actual Concepts disallowing valid use
cases. Therefore the code before 5f2560f would perhaps have worked on the
compilers you were using but would have failed elsewhere. Let's see if we
can get it working everywhere.

My assumption is that by a minor edit to boost/range/config.hpp you can get
back up and running. Please confirm/deny this assumption.

> Any chance to get this fixed for 1.56.0 ?
>
>
I shall try to within the constraints of the release schedule. It is
starting to get a little tight. Hopefully this will only require changes
that are backward compatible. I certainly shall provide you with a solution
although it might require a small patch if I miss the release window.

> Yours,
>
> Jürgen
>

Sorry for the inconvenience caused by these changes. I hope I can convince
you that it is part of a march toward a general improvement so that users
can more frequently enjoy the benefits of concept checking.

Regards,
Neil Groves