Is it possible to somehow contain an extern "C" function within a header file?

Due to templates being templates, they need to be defined in a header file (making explicit instantiations is not an option). I have an extern function which takes an expanded template parameter pack:

#pragma once

extern "C" int bar(...);

class Foo {
    template <typename ...Args>
    int foo(Args&&... args) {
        return bar(args...);
    }
};

I thought about it for quite a while, and I can't figure out how to decouple this. I don't want bar to be in the header and the consumers of said header to have access to it, but because templates must be in a header file, I can't make a wrapper function that would contain the extern. The extern function cannot take va_list.

Is this even possible to do in a portable (compiler agnostic) way?

PS: the extern could be really anywhere, as long as it would somehow bypass templates requirement to be withing the header, and not be accessible to consumers of the header.

Language specifications can only appear in namespace scope, so you cannot restrict the declaration to the function :-/

One workaround which should work (maybe except for Oracle Studio See note #1) would be to use function pointer:

class Foo
{
    static int (* const fbar) (...);
public:

    template <typename ...Args>
    int foo(Args&&... args) {
        return fbar(args...);
    }
};

and in cpp:

extern "C" int bar(...);

int (* const Foo::fbar)(...) = &bar;

Note #1:

According to cppreference

The only modern compiler that differentiates function types with "C" and "C++" language linkages is Oracle Studio [..]

So I'm unsure it would work for that compiler directly (maybe a extra cast would be required).