module base;
import derived;
class Base {
    private void foo() { }
    void callFoo(Derived d) { d.foo(); }
}
-------
module derived;
import base;
class Derived : Base { }
-------
import base;
import derived;
int main(string[] argv) {
    auto d = new Derived();
    d.callFoo(new Derived());
}

error:
base.d(6): Deprecation: base.Base.foo is not visible from class Derived

The method d.foo() should be callable from inside 'callFoo'. Casting 'd' to 'Base' inside 'callFoo()' before invoking 'foo' suppresses the error.