What is the static type of 'this'

In the following code, I make a call to func() that accepts Base, but
I pass DerivedFromPrivateBase which privately inherits Base.
The call is accepted (compiles) when made from a member of
DerivedFromPrivateBase f2.
Does not compile if called from elsewhere, say from
TestPrivateInheritance(). Is this OK?
Compiled on Visual C++ 2008 express and Comeau
---------------------------------

#include <iostream>
using namespace std;

struct Base
{
    virtual void f() = 0;
};

void func(Base* b)
{
    b->f(); // calls DerivedFromPrivateBase ::f() in the code
}

struct DerivedFromPrivateBase : private Base
{
    void f()
    {
        cout << "DerivedFromPrivateBase:f() called" << endl;
    }
    void f2()
    {
        func(this); // Compiles: this is DerivedFromPrivateBase but calls
func that accepts Base
    }
};

void TestPrivateInheritance()
{
    DerivedFromPrivateBase d;
    d.f2();
    // func (&d); // this gives compile error about inaccessible base
}

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