This is a wrong D2 program:


immutable class Foo {}
class Bar : Foo { int x; }
void main() {
    auto b = new Bar;
    b.x = 10; // line 5
}


From the error message it's clear that x is not a mutable variable:
test.d(5): Error: can only initialize const member x inside constructor

But from the code of the Foo class there is no clear indication that x is immutable. So in this situation I think it's better if the compiler requires a immutable annotation on x too, for code readability, something like:


immutable class Foo {}
class Bar : Foo { int x; } // Error: attribute x requires 'immutable' annotation
void main() {
    auto b = new Bar;
}



immutable class Foo {}
class Bar : Foo { immutable int x; } // OK, no error
void main() {
    auto b = new Bar;
}