[boost] [assign] [type_traits] Bug with is_convertible + list_of

From: Edson Tadeu (e.tadeu_at_[hidden])
Date: 2007-08-20 07:09:36

Please take a look at the following small test code:

/*  1 */  #include <boost/assign/list_of.hpp>
/*  2 */  #include <boost/type_traits/is_convertible.hpp>
/*  3 */  #include <iostream>
/*  4 */
/*  5 */  template <class T>
/*  6 */  bool test1(T value) {
/*  7 */      return boost::is_convertible<T, int>::value;
/*  8 */  }
/*  9 */
/* 10 */  template <class T>
/* 11 */  int test2(T value) {
/* 12 */      return static_cast<int>(value);
/* 13 */  }
/* 14 */
/* 15 */
/* 16 */  void main() {
/* 17 */      std::cout << test1(2.0) << ", ";
/* 18 */      std::cout << test1("Unconvertible") << ", ";
/* 19 */      std::cout << test1(boost::assign::list_of(1)(3)) << ", ";
/* 20 */
/* 21 */      std::cout << test2(2.0) << std::endl;
/* 22 */      // std::cout << test2(boost::assign::list_of(1)(3)) <<
std::endl;
/* 23 */  }

When I try to use is_convertible<T, int>::value, with T being a
boost::assign::list_of, it wrongly results in a "true" value. This code
compiles on MSVC 8.0, and running it gives the following output:

1, 0, 1, 2

If you uncomment line 22 (i.e., if you *really* try to convert the types),
the code will not compile, with an error on line 19:

boost-1.33.1\include\boost\assign\list_of.hpp(152) : error C2564: 'int' : a
function-style conversion to a built-in type can only take one argument
        boost-1.33.1\include\boost\assign\list_of.hpp(129) : see reference
to function template instantiation 'Container
boost::assign_detail::converter<DerivedTAssign>::convert<Container>(const
Container *,boost::assign_detail::default_type_tag) const' being compiled
        with
        [
            Container=int,
            DerivedTAssign=boost::assign_detail::generic_list<int>
        ]
        boost-1.33.1\include\boost\assign\list_of.hpp(336) : see reference
to function template instantiation 'Container
boost::assign_detail::converter<DerivedTAssign>::convert_to_container<Container>(void)
const' being compiled
        with
        [
            Container=int,
            DerivedTAssign=boost::assign_detail::generic_list<int>
        ]
        boost-1.33.1\include\boost\type_traits\is_convertible.hpp(202) : see
reference to function template instantiation
'boost::assign_detail::generic_list<T>::operator Container(void) const<To>'
being compiled
        with
        [
            T=int,
            Container=int,
            To=int
        ]
        boost-1.33.1\include\boost\type_traits\is_convertible.hpp(228) : see
reference to class template instantiation
'boost::detail::is_convertible_basic_impl<From,To>' being compiled
        with
        [
            From=boost::assign_detail::generic_list<int> &,
            To=int
        ]
        boost-1.33.1\include\boost\type_traits\is_convertible.hpp(302) : see
reference to class template instantiation
'boost::detail::is_convertible_impl<From,To>' being compiled
        with
        [
            From=boost::assign_detail::generic_list<int>,
            To=int
        ]
        boost-1.33.1\include\boost\type_traits\is_convertible.hpp(348) : see
reference to class template instantiation
'boost::detail::is_convertible_impl_dispatch<From,To>' being compiled
        with
        [
            From=boost::assign_detail::generic_list<int>,
            To=int
        ]
        test_is_convertible.cpp(7) : see reference to class template
instantiation 'boost::is_convertible<From,To>' being compiled
        with
        [
            From=boost::assign_detail::generic_list<int>,
            To=int
        ]
        test_is_convertible.cpp(19) : see reference to function template
instantiation 'bool
test1<boost::assign_detail::generic_list<T>>(boost::assign_detail::generic_list<T>)'
being compiled
        with
        [
            T=int
        ]