decltype expressions involving polymorphic callable entities fails in VC++11 Beta but works in VC++10 by sharp_kid

Writing decltype expressions involving polymorphic callable entities (template function call operator overload) in VC++11 Beta causes the compile-error:

"error C2893: Failed to specialize function template ''unknown-type'..."

But the same decltype expressions compiles and works fine in VC++2010 (and GCC 4.6.2). Code which can reproduce the problem:

template < typename Fn >
struct Foo
{
    Fn fn;

    template < typename Tp >
    auto operator()(const Tp& x) const -> decltype(fn(x)) { return fn(x); }
};

struct Bar
{
    template < typename Tp >
    int operator()(const Tp& x) const { return 0; }
};

template < typename Fn >
void foobar(Fn f)
{
    typedef decltype(f(0)) result_type;
}

int main()
{
    foobar(Foo<Bar>());
    return 0;
}