MSVC C4180 warning

[Kojoley]

Code:

#include <boost/spirit/include/qi_parse.hpp>
#include <boost/spirit/include/qi_action.hpp>
#include <boost/spirit/include/qi_numeric.hpp>

void fun(int) {}

int main()
{
    namespace qi = boost::spirit::qi;
    char const *s = "42", *e = s + std::strlen(s);
    qi::parse(s, e, qi::int_[fun]);
}

Warning message:

1>boost/proto/extends.hpp(552): warning C4180: qualifier applied to function type has no meaning; ignored
1>main.cpp(11): note: see reference to function template instantiation 'const boost::proto::exprns_::expr<Tag,Args,2> boost::proto::exprns_::extends<boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<T>,0>,boost::spirit::terminal<boost::spirit::tag::int_>,boost::proto::domainns_::default_domain,0>::operator []<void(int)>(A (__cdecl &)) const' being compiled
1>        with
1>        [
1>            Tag=boost::proto::tagns_::tag::subscript,
1>            Args=boost::proto::argsns_::list2<const boost::spirit::terminal<boost::spirit::tag::int_> &,boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<void (__cdecl &)(int)>,0>>,
1>            T=boost::spirit::tag::int_,
1>            A=void (int)
1>        ]
1>main.cpp(11): note: see reference to function template instantiation 'const boost::proto::exprns_::expr<Tag,Args,2> boost::proto::exprns_::extends<boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<T>,0>,boost::spirit::terminal<boost::spirit::tag::int_>,boost::proto::domainns_::default_domain,0>::operator []<void(int)>(A (__cdecl &)) const' being compiled
1>        with
1>        [
1>            Tag=boost::proto::tagns_::tag::subscript,
1>            Args=boost::proto::argsns_::list2<const boost::spirit::terminal<boost::spirit::tag::int_> &,boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<void (__cdecl &)(int)>,0>>,
1>            T=boost::spirit::tag::int_,
1>            A=void (int)
1>        ]

MWE:

#define BOOST_PROTO_DONT_USE_PREPROCESSED_FILES
#include <boost/proto/proto.hpp>

struct placeholder {};

void fun() {}

namespace proto = boost::proto;

template<typename Expr>
struct eterm : proto::extends<Expr, eterm<Expr> >
{
    typedef proto::extends<Expr, eterm<Expr> > base_type;
    
    eterm(Expr const& expr = Expr())
        : base_type(expr)
    {}

    BOOST_PROTO_EXTENDS_USING_ASSIGN(eterm)
};

int main()
{
    {
        proto::terminal<placeholder>::type term;
        term[fun]; // produces C4180 warning
        term(fun); // produces C4180 warning
        term = fun; // produces C4180 warning
        term + fun;  // fine
    }
    {
        eterm<proto::terminal<placeholder>::type> term;
        term[fun]; // produces C4180 warning
        term(fun); // produces C4180 warning
        term = fun; // produces C4180 warning
        term + fun;  // fine
    }
}