java - instanceof operator - why there is Illegal compile time error

Question

Considering the following code, I don't understand why "System.out.println( c2 instanceof D);" will result an "illegal compile time error" but not return "false"? Many thanks for your help!

interface I { }
class A { int x = 1;}
class B extends A implements I { int y = 2;}
class C extends B { }
class D extends B{ }
class E implements I { }
C c2 = new C();`

Answer 1

Because compiler knows that casting c2 to type D will always doing to fail at runtime, and hence is marked as compile time error. So it doesn't allow such instanceof to pass through.

Quoting JLS §15.20.2:

If a cast (§15.16) of the RelationalExpression to the ReferenceType would be rejected as a compile-time error, then the instanceof relational expression likewise produces a compile-time error. In such a situation, the result of the instanceof expression could never be true.