Summary

The javac compiler is accepting invalid programs which according to the JLS 23 should be rejected. The javac compiler is not in sync with section §5.1.6.1 of the JSL 23. In particular the javac compiler is not able to detect in some cases that a freely extensible class is disjoint from a sealed interface.

Problem

The javac compiler is not in sync with section §5.1.6.1 of the JLS 23. This implies that code like:

class Test {
    sealed interface I permits C1 {}
    non-sealed class C1 implements I {}
    class C3 {}
    I m(int s, C3 c3) {
        I i = (I)c3;
    }
}

is accepted by the javac compiler even when according to the JLS 23 class C3 and interface I are disjoint as C3 is a freely extensible class and it is disjoint from all the permitted subclasses of interface I.

Solution

The proposed solution is to sync the javac compiler with section §5.1.6.1 of the JSL 23. In particular the algorithm to determine if two types are disjoint should be improved to correctly determine if a freely extensible class is disjoint or not from a sealed interface.

Specification

Of note for this CSR are section §5.1.6 Narrowing Reference Conversion and subsection §5.1.6.1 Allowed Narrowing Reference Conversion of the JLS 23. When determining if a narrowing reference conversion exist from a type S to a type T, see §5.1.6 there are cases for which it should be determined if two types are disjoint or not. The rules for disjointess are defined in subsection §5.1.6.1. We are in particular interested in this text of §5.1.6.1:

• A class named C is disjoint from an interface named I if (i) it is not the case
  that C <: I , and (ii) one of the following cases applies:
    – C is final .
    – C is sealed , and all of the permitted direct subclasses of C are disjoint
      from I .
    – C is freely extensible (§8.1.1.2), and I is sealed , and C is disjoint from
      all of the permitted direct subclasses and subinterfaces of I.

...

• A class named C is disjoint from another class named D if (i) it is not the case
  that C <: D , and (ii) it is not the case that D <: C .