MSVC C4180 warning

Code:
```cpp
#include
#include
#include

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 boost::proto::exprns_::extends,0>,boost::spirit::terminal,boost::proto::domainns_::default_domain,0>::operator [](A (__cdecl &)) const' being compiled
1> with
1> [
1> Tag=boost::proto::tagns_::tag::subscript,
1> Args=boost::proto::argsns_::list2 &,boost::proto::exprns_::expr,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 boost::proto::exprns_::extends,0>,boost::spirit::terminal,boost::proto::domainns_::default_domain,0>::operator [](A (__cdecl &)) const' being compiled
1> with
1> [
1> Tag=boost::proto::tagns_::tag::subscript,
1> Args=boost::proto::argsns_::list2 &,boost::proto::exprns_::expr,0>>,
1> T=boost::spirit::tag::int_,
1> A=void (int)
1> ]
```

MWE:
```cpp
#define BOOST_PROTO_DONT_USE_PREPROCESSED_FILES
#include

struct placeholder {};

void fun() {}

namespace proto = boost::proto;

template
struct eterm : proto::extends >
{
typedef proto::extends > base_type;

eterm(Expr const& expr = Expr())
: base_type(expr)
{}

BOOST_PROTO_EXTENDS_USING_ASSIGN(eterm)
};

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