warn_no_constructor_for_refconst can not be disabled (Boost.ConceptCheck)

[mloskot]

Bugzilla Link	41759
Version	unspecified
OS	Linux
CC	@dwblaikie,@DougGregor,@mizvekov,@zygoloid

Extended Description

The warn_no_constructor_for_refconst (and possibly other from close category) can not be disabled as there is no corresponding -Wno flag available in clang++<=9.

This causes issues while compiling code that makes use of Boost.ConceptCheck library. For example:

#include <boost/concept_check.hpp>
int main()
{
    using T = int const&;
    boost::function_requires
        <
            boost::EqualityComparableConcept<T>
        >();
}

will trigger a flood of warnings:

$ clang++ -std=c++11 -I/mnt/d/boost.wsl clang_warn_no_constructor_for_refconst.cpp
In file included from clang_warn_no_constructor_for_refconst.cpp:1:
/mnt/d/boost.wsl/boost/concept_check.hpp:231:17: warning: struct 'EqualityComparable<const int &>' does not declare any constructor
      to initialize its non-modifiable members
  BOOST_concept(EqualityComparable,(TT))
                ^
/mnt/d/boost.wsl/boost/concept_check.hpp:231:17: note: in instantiation of template class 'boost::EqualityComparable<const int &>'
      requested here
/mnt/d/boost.wsl/boost/concept/detail/has_constraints.hpp:32:63: note: in instantiation of template class
      'boost::EqualityComparableConcept<const int &>' requested here
  inline yes has_constraints_(Model*, wrap_constraints<Model,&Model::constraints>* = 0);
                                                              ^
/mnt/d/boost.wsl/boost/concept/detail/has_constraints.hpp:44:25: note: while substituting deduced template arguments into function
      template 'has_constraints_' [with Model = boost::EqualityComparableConcept<const int &>]
      , value = sizeof( detail::has_constraints_((Model*)0) ) == sizeof(detail::yes) );
                        ^
/mnt/d/boost.wsl/boost/concept/detail/general.hpp:53:19: note: in instantiation of template class
      'boost::concepts::not_satisfied<boost::EqualityComparableConcept<const int &> >' requested here
        concepts::not_satisfied<Model>::value
                  ^
/mnt/d/boost.wsl/boost/concept_check.hpp:50:7: note: in instantiation of template class 'boost::concepts::requirement_<void
      (*)(boost::EqualityComparableConcept<const int &>)>' requested here
      BOOST_CONCEPT_ASSERT((Model));
      ^
/mnt/d/boost.wsl/boost/concept/assert.hpp:43:5: note: expanded from macro 'BOOST_CONCEPT_ASSERT'
    BOOST_CONCEPT_ASSERT_FN(void(*)ModelInParens)
    ^
/mnt/d/boost.wsl/boost/concept/detail/general.hpp:71:25: note: expanded from macro 'BOOST_CONCEPT_ASSERT_FN'
    &::boost::concepts::requirement_<ModelFnPtr>::failed>    \
                        ^
clang_warn_no_constructor_for_refconst.cpp:6:12: note: in instantiation of function template specialization
      'boost::function_requires<boost::EqualityComparableConcept<const int &> >' requested here
    boost::function_requires
           ^
/mnt/d/boost.wsl/boost/concept_check.hpp:238:8: note: reference member 'a' will never be initialized
    TT a, b;
       ^
/mnt/d/boost.wsl/boost/concept_check.hpp:238:11: note: reference member 'b' will never be initialized
    TT a, b;
          ^
1 warning generated.

There is no way to disable this

warning: struct 'EqualityComparable<const int &>' does not declare any constructor to initialize its non-modifiable members

Although I understand and agree with rationale provided by Chandler here https://softwareengineering.stackexchange.com/questions/122608/clang-warning-flags-for-objective-c-development/124574#124574, to warning about "never really correct" code, I think that in this particular use case of the Boost.ConceptCheck library, it is Okey to expect to be able to disable the warn_no_constructor_for_refconst (and similar).

The PITA is that for any non-trivial size of templated library that makes use of Boost.ConceptCheck, compilation with Clang leads to a flood of warning.
As side effect, it causes trouble when compiling on popular CI services available for open source projects (timeouts, builds killed due to logs too long etc.)

[mizvekov]

I failed to reproduce this on compiler explorer.

If this is an issue only with old clang on old boost, I am not sure we can really do anything about it.

If you have more details or a standalone repro please reopen.

[mloskot]

Screenshot with repro of the issue using clang 12 and Boost 1.77 (develop branch)
I can still reproduce the issue using clang 12 and Boost 1.77 (current develop branch):

$ clang++ --version | grep version
Ubuntu clang version 12.0.1-++20210630032618+fed41342a82f-1~exp1~20210630133332.127

$ grep '#define BOOST_VERSION ' /home/mloskot/boost/boost/version.hpp
#define BOOST_VERSION 107700

$ cat test.cpp

#include <boost/concept_check.hpp>
int main()
{
    using T = int const&;
    boost::function_requires<boost::EqualityComparableConcept<T>>();
}

$ clang++ -std=c++11 -I/home/mloskot/boost test.cpp
In file included from test.cpp:1:
/home/mloskot/boost/boost/concept_check.hpp:231:17: warning: struct 'EqualityComparable<const int &>' does not declare any constructor to initialize its non-modifiable members
  BOOST_concept(EqualityComparable,(TT))
                ^
/home/mloskot/boost/boost/concept_check.hpp:231:17: note: in instantiation of template class 'boost::EqualityComparable<const int &>' requested here
/home/mloskot/boost/boost/concept/detail/has_constraints.hpp:32:63: note: in instantiation of template class 'boost::EqualityComparableConcept<const int &>' requested here
  inline yes has_constraints_(Model*, wrap_constraints<Model,&Model::constraints>* = 0);
                                                              ^
/home/mloskot/boost/boost/concept/detail/has_constraints.hpp:44:25: note: while substituting deduced template arguments into function template 'has_constraints_' [with Model = boost::EqualityComparableConcept<const int &>]
      , value = sizeof( detail::has_constraints_((Model*)0) ) == sizeof(detail::yes) );
                        ^
/home/mloskot/boost/boost/concept/detail/general.hpp:74:19: note: in instantiation of template class 'boost::concepts::not_satisfied<boost::EqualityComparableConcept<const int &>>' requested here
        concepts::not_satisfied<Model>::value
                  ^
/home/mloskot/boost/boost/concept_check.hpp:50:7: note: in instantiation of template class 'boost::concepts::requirement_<void (*)(boost::EqualityComparableConcept<const int &>)>' requested here
      BOOST_CONCEPT_ASSERT((Model));
      ^
/home/mloskot/boost/boost/concept/assert.hpp:43:5: note: expanded from macro 'BOOST_CONCEPT_ASSERT'
    BOOST_CONCEPT_ASSERT_FN(void(*)ModelInParens)
    ^
/home/mloskot/boost/boost/concept/detail/general.hpp:92:25: note: expanded from macro 'BOOST_CONCEPT_ASSERT_FN'
    &::boost::concepts::requirement_<ModelFnPtr>::failed>    \
                        ^
test.cpp:5:12: note: in instantiation of function template specialization 'boost::function_requires<boost::EqualityComparableConcept<const int &>>' requested here
    boost::function_requires<boost::EqualityComparableConcept<T>>();
           ^
/home/mloskot/boost/boost/concept_check.hpp:238:8: note: reference member 'a' will never be initialized
    TT a, b;
       ^
/home/mloskot/boost/boost/concept_check.hpp:238:11: note: reference member 'b' will never be initialized
    TT a, b;
          ^
1 warning generated.

[mloskot]

@​Matheus Izvekov

I asked here https://cpplang.slack.com/archives/C28NX7947/p1627297817121500
about why the Compiler Explorer is not showing the error and Matt Godbolt tweaked the command line, and the CE now also can reproduce the problem, see:

https://godbolt.org/z/os4YGMoe3

So, I've reopened the bug.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Mateusz Łoskot (mloskot)

| | | | --- | --- | | Bugzilla Link | [41759](https://llvm.org/bz41759) | | Version | unspecified | | OS | Linux | | CC | @dwblaikie,@DougGregor,@mizvekov,@zygoloid |

Extended Description

The warn_no_constructor_for_refconst (and possibly other from close category) can not be disabled as there is no corresponding -Wno flag available in clang++<=9.

This causes issues while compiling code that makes use of Boost.ConceptCheck library. For example:

#include <boost/concept_check.hpp>
int main()
{
    using T = int const&;
    boost::function_requires
        <
            boost::EqualityComparableConcept<T>
        >();
}

will trigger a flood of warnings:

$ clang++ -std=c++11 -I/mnt/d/boost.wsl clang_warn_no_constructor_for_refconst.cpp
In file included from clang_warn_no_constructor_for_refconst.cpp:1:
/mnt/d/boost.wsl/boost/concept_check.hpp:231:17: warning: struct 'EqualityComparable<const int &>' does not declare any constructor
      to initialize its non-modifiable members
  BOOST_concept(EqualityComparable,(TT))
                ^
/mnt/d/boost.wsl/boost/concept_check.hpp:231:17: note: in instantiation of template class 'boost::EqualityComparable<const int &>'
      requested here
/mnt/d/boost.wsl/boost/concept/detail/has_constraints.hpp:32:63: note: in instantiation of template class
      'boost::EqualityComparableConcept<const int &>' requested here
  inline yes has_constraints_(Model*, wrap_constraints<Model,&Model::constraints>* = 0);
                                                              ^
/mnt/d/boost.wsl/boost/concept/detail/has_constraints.hpp:44:25: note: while substituting deduced template arguments into function
      template 'has_constraints_' [with Model = boost::EqualityComparableConcept<const int &>]
      , value = sizeof( detail::has_constraints_((Model*)0) ) == sizeof(detail::yes) );
                        ^
/mnt/d/boost.wsl/boost/concept/detail/general.hpp:53:19: note: in instantiation of template class
      'boost::concepts::not_satisfied<boost::EqualityComparableConcept<const int &> >' requested here
        concepts::not_satisfied<Model>::value
                  ^
/mnt/d/boost.wsl/boost/concept_check.hpp:50:7: note: in instantiation of template class 'boost::concepts::requirement_<void
      (*)(boost::EqualityComparableConcept<const int &>)>' requested here
      BOOST_CONCEPT_ASSERT((Model));
      ^
/mnt/d/boost.wsl/boost/concept/assert.hpp:43:5: note: expanded from macro 'BOOST_CONCEPT_ASSERT'
    BOOST_CONCEPT_ASSERT_FN(void(*)ModelInParens)
    ^
/mnt/d/boost.wsl/boost/concept/detail/general.hpp:71:25: note: expanded from macro 'BOOST_CONCEPT_ASSERT_FN'
    &::boost::concepts::requirement_<ModelFnPtr>::failed>    \
                        ^
clang_warn_no_constructor_for_refconst.cpp:6:12: note: in instantiation of function template specialization
      'boost::function_requires<boost::EqualityComparableConcept<const int &> >' requested here
    boost::function_requires
           ^
/mnt/d/boost.wsl/boost/concept_check.hpp:238:8: note: reference member 'a' will never be initialized
    TT a, b;
       ^
/mnt/d/boost.wsl/boost/concept_check.hpp:238:11: note: reference member 'b' will never be initialized
    TT a, b;
          ^
1 warning generated.

There is no way to disable this

warning: struct 'EqualityComparable<const int &>' does not declare any constructor to initialize its non-modifiable members

Although I understand and agree with rationale provided by Chandler here https://softwareengineering.stackexchange.com/questions/122608/clang-warning-flags-for-objective-c-development/124574#124574, to warning about "never really correct" code, I think that in this particular use case of the Boost.ConceptCheck library, it is Okey to expect to be able to disable the warn_no_constructor_for_refconst (and similar).

The PITA is that for any non-trivial size of templated library that makes use of Boost.ConceptCheck, compilation with Clang leads to a flood of warning.
As side effect, it causes trouble when compiling on popular CI services available for open source projects (timeouts, builds killed due to logs too long etc.)