id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
7120	ambiguous overload of convert_construct in variant when one of variant's value types derives from variant	jeffrey.hellrung	Antony Polukhin	"#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?"	Bugs	closed	Boost 1.64.0	variant	Boost 1.51.0	Problem	fixed