Weird "nested function cannot be accessed" error

[Max Samukha via Digitalmars-d-learn]

This code fails to compile:

void bar(alias f)() {
    f();
}
void foo(alias f)() {
    bar!f();
}

void main() {
    void f()() {
    }
    foo!f();
}

Error: function test.main.f!().f is a nested function and cannot 
be accessed from test.bar!(f).bar


But non-template nested functions are accepted:

void main() {
    void f() {
    }
    foo!f(); // ok
}


What's going on here?