Casting subtype fails - dart

Question

class A {
void hello() {
print('world');
}
}
class B extends A {
#override
void hello() {
print('StackOverflow');
}
}
(A() as B).hello(); results in type 'A' is not a subtype of type 'B' in type cast.

Answer 1

Full disclosure: I don't know Dart.
You can't do this kind of cast because it might result in function calls or field accesses that aren't well defined.
I'll change your example to demonstrate:
class A {
void hello() {
print('world');
}
}
class B extends A {
#override
void hello() {
print('StackOverflow');
}
void hello2() {
print('Is Great!');
}
}
Now if you do (A() as B).hello2(); what should Dart do? This isn't really obvious, so it doesn't allow you to do that. Going the other way isn't an issue since B inherits all stuff from A.
class A {
void hello() {
print('world');
}
}
class B extends A {
#override
void hello() {
print('StackOverflow');
}
}
class C extends A {
#override
void hello() {
print('Hello');
}
}
Another issue is a value that has type A may be a different subtype of A like C

Answer 2

The way casting works is you can only go from a more specific type, B in this case, to a more general type, A. Your creating an instance of A, but A is not B.

Answer 3

Dart generally allows you to down-cast from a super-type to a sub-type because the value might actually be of the sub-type.
Animal animal = Cat();
if (something) animal = Dog();
...
Cat cat = animal as Cat; // Allowed, the animal may be a cat.
However, Dart 2 disallows down-casts in a few cases where it is obvious (even to the compiler) that the cast will always fail at run-time. That's what you are hitting here: (A() as B). The A() calls a generative constructor, so the compiler knows that the type of that expressions is exactly A (and not any proper sub-type of A). So, it knows that casting that to B will always fail at run-time, and for your own protection, it disallows the program entirely. Hence, a compile-time error.