class Base
{
    int _val;
}

class Foo : Base
{
    Base base() const { return this; }
}

void main()
{
    const(Foo) foo = new const(Foo);
    static assert(!__traits(compiles, foo._val = 1));
    static assert(!__traits(compiles, foo.base()._val = 1));
    foo.base()._val = 1;
    assert(foo._val == 0);
}

Also broken for immutable and shared.
--------

This is probably implemented as 'return cast(Base)this;' and
needs to include the right modifiers.