If a method of a class returns a const or immutable value, and
the class has class invariant, then the compile fails.  It also
fails if a method returning a const/immutable has an out contract.

The following code:
--------------------
class Foo
{
    invariant() {}
    const(int) foo()
    {
        return 0;
    }
    immutable(int) bar()
    out(abc) { }
    body
    {
        return 0;
    }
}
--------------------
causes this compile error:
--------------------
test.d(4): Error: variable test.Foo.foo.__result cannot modify const
test.d(9): Error: variable test.Foo.bar.abc cannot modify mutable
--------------------