[Boost-bugs] [Boost C++ Libraries] #7120: ambiguous overload of convert_construct in variant when one of variant's value types derives from variant

Subject: [Boost-bugs] [Boost C++ Libraries] #7120: ambiguous overload of convert_construct in variant when one of variant's value types derives from variant
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2012-07-11 20:21:38

#7120: ambiguous overload of convert_construct in variant when one of variant's
value types derives from variant
------------------------------+---------------------------------------------
 Reporter:  jeffrey.hellrung  |       Owner:  ebf    
     Type:  Bugs              |      Status:  new    
Milestone:  To Be Determined  |   Component:  variant
  Version:  Boost 1.51.0      |    Severity:  Problem
 Keywords:                    |  
------------------------------+---------------------------------------------
 #include <boost/variant.hpp>

 struct X
     : boost::variant< int >
 { };

 void main()
 {
     X x;
     boost::variant<X> y(x);
 }

 yields (MSVC9)

 1>------ Build started: Project: scratch, Configuration: Debug Win32
 ------
 1>Compiling...
 1>main.cpp
 1>c:\users\jeffrey\boost_1_49_0\boost\variant\variant.hpp(1405) : error
 C2666: 'boost::variant<T0_>::convert_construct' : 3 overloads have similar
 conversions
 1>        with
 1>        [
 1>            T0_=X
 1>        ]
 1>        c:\users\jeffrey\boost_1_49_0\boost\variant\variant.hpp(1384):
 could be 'void
 boost::variant<T0_>::convert_construct<int,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_>(const
 boost::variant<int> &,long)'
 1>        with
 1>        [
 1>            T0_=X
 1>        ]
 1>        c:\users\jeffrey\boost_1_49_0\boost\variant\variant.hpp(1375):
 or       'void
 boost::variant<T0_>::convert_construct<int,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_>(boost::variant<int>
 &,long)'
 1>        with
 1>        [
 1>            T0_=X
 1>        ]
 1>        c:\users\jeffrey\boost_1_49_0\boost\variant\variant.hpp(1315):
 or       'void boost::variant<T0_>::convert_construct<T>(T
 &,int,boost::mpl::false_)'
 1>        with
 1>        [
 1>            T0_=X,
 1>            T=X
 1>        ]
 1>        while trying to match the argument list '(X, long)'
 1>        c:\users\jeffrey\scratch\main.cpp(10) : see reference to
 function template instantiation 'boost::variant<T0_>::variant<X>(T &)'
 being compiled
 1>        with
 1>        [
 1>            T0_=X,
 1>            T=X
 1>        ]

 Basically, the problem appears to be the call to

 convert_construct(operand, 1L) // operand is of type X

 which cannot disambiguate between the overloads (simplified)

 void convert_construct([const] boost::variant<T>& operand, long);
 void convert_construct(T& operand, int);

 The former requires a derived->base conversion in the first argument,
 while the latter requires a long->int conversion in the second argument.
 Perhaps the dispatching among the convert_construct overloads should be
 more sophisticated?

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/7120>
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