Casts that makes no sense (!)

[Ary Borenszweig]

The following compiles:

import std.stdio;
interface I {
}

class A : I {
}

class B {
}

int main() {
  I i = new A();
  A a = cast(A) i;
  B b = cast(B) i; // shouldn't compile
  B c = cast(B) a; // shouldn't compile

  writeln(a);
  writeln(b);
  writeln(c);

  return 0;
}

But two lines there doesn't make sense:

B b = cast(B) i;

An instance of I can never be a B, so why the cast is allowed?

B c = cast(B) a;

An instance of A can never be an A, so why the cast is allowed?

I think these casts should result in an error. This can prevent some bugs.

Java and C# work like that. You can't cast an object of instance of type 
A to type B if both types are classes and B isn't a supertype or subtype 
of A.