id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
4540	fold<Sequence>() instantiates result_of::fold<Sequence const>	anonymous	Douglas Gregor	"Hi,

this is a complicated one, I'll try my best:

there is a problem with fold() and maybe other fusion functions whose result is computed from the result type of user functors.

in iteration/detail/fold.hpp, there are two fold()'s, the default one and a specialization for const sequences:
{{{
    inline typename result_of::fold<Seq const,State const,F>::type
    BOOST_FUSION_FOLD_NAME(Seq const& seq,State const& state,F f)
}}}
I'm not sure why it is there since there isn't any difference to the non-const fold().
but the fact that it is there causes problems when using fold() with a non-const sequence:

{{{
    fusion::vector<int> vec;
    fusion::fold(vec,0,F());
}}}
even though the sequence is not const, at least MSVC (and I vaguely remember something about that in the C++ standard) instantiates all result types of the function overload set, before the arguments are matched to the non-const fold().

==> fusion::result_of::fold<Sequence '''const''',State,F> is instantiated.

under normal circumstances this only constitutes an unnecessary instantiation, but when the result type depends on user functors it can cause instantiatiation of undefined types.

for example:
{{{
struct F{
    template<typename Args>
    struct result;
    int operator()(int state,int &element) const{
        return state;
    }
};

template<typename State>
struct F::result<F(State,int &)>{
    typedef int type;
};

int main(){
    fusion::vector<int> vec;
    fusion::fold(vec,0,F());
}
}}}

the result type of F is only defined for argument type ""int &"", which is enough for this fold. but the call to fold() instantiates the ''const'' specialization of fold(), and therefore instantiates result_of::fold<vector '''const'''>, which instantiates the result type of F(State,int const &) ==> compiler error

when the following code snippet is added as a workaround, it works:

{{{
template<typename State>
struct F::result<F(State,int const &)>{
    typedef int type;
};
}}}

bottom line - the const specialization of fold() probably shouldn't be there.
"	Feature Requests	new	Boost 1.44.0	result_of	Boost Development Trunk	Problem			mr.chr.schmidt@…