How to use pimpl idiom with templated functions

Question

I have a class which has a template member functions and the private members needs to be put in the implementation class by pimpl idiom approach.

Foo.hpp

#include <iostream>
class Foo
{
public:
private:
    class FooImpl;
    std::unique_ptr<FooImpl> impl_;

public:
    Foo() = default;
    template <class F, class... Args>
    auto SomeFunction ( F &&f, Args &&...args) -> boost::unique_future<decltype (f(args...))>;
};

Foo.cpp

#include <Foo.hpp>
#include <FooImpl.hpp>
template <class... Args>
template <class F, class... Args>
auto SomeFunction ( F &&f, Args &&...args) -> boost::unique_future<decltype (f(args...))>
{
   impl->SomeFunction(std::forward<F>(f),std::forward<Args>(args)...);
}

FooImpl.hpp

#include <Foo.hpp>
class Foo::FooImpl
{
public:
    FooImpl() = default;
    template <class F, class... Args>
    auto SomeFunction ( F &&f, Args &&...args) -> boost::unique_future<decltype (f(args...))>;
private:
    int dummy;
};

FooImpl.cpp

#include <FooImpl.hpp>
template <class... Args>
template <class F, class... Args>
auto SomeFunction ( F &&f, Args &&...args) -> boost::unique_future<decltype (f(args...))>
{
   //Do Something...
}

I have tried the following approaches but nothing worked in my case:-

1. With virtual functions. This didn't work because virtual functions cannot be templated.
2. Explicitly instantiate all the template instances like

template class Foo<int>;
template class Foo<float>;

This won't suit in my case because any type can come in my function. I cannot explicitly instantiate all types.

Is there other approaches? Is it possible to implement pimpl idiom with template functions? Or is there any other alternative to pimpl idiom?