That doesn't mean anything, really. Eclipse's compiler does not guarantee 1-to-1 correctness/equivalence with `javac` or any other compiler, so it's entirely possible that Eclipse is wrongfully rejecting this example, when it should actually accept it like `javac` does. So it is a "compiler bug", but in the other direction.

The paper actually explicitly acknowledges this, Section 3 pg. 3, if you read closely. In fact there is an even bigger danger lurking, which is that Java has non-deterministic type inference. It's not even necessarily a "compiler bug" that the first example, Unsound.java, is rejected:

> ... As a consequence, the method type-checks. Furthermore, even though type-argument inference in Java is undecidable, this type argument is identified by javac, version 1.8.0_25, which consequently accepts and compiles the code in Figure 1. However, constraint solving is generally a non-deterministic process, and type-argument inference itself is non-deterministic [35], so the Eclipse Compiler for Java, ecj version 3.11.1.v20150902-1521, and the current compiler for the upcoming Java 9, javac build 108, fail to type-check this code...

Non-determinism means something very specific here: it does not necessarily mean the compiler randomly rejects something based on whether it's a random day of the week or because `rand() % 4 == 0`. It means the compiler has no sound method for inferring the type of a value, given multiple choices, so it "just" picks one, in an ill specified way. It could pick the first one or the second, and whichever type it picks will influence further constraint generation/inference. See this paper, section 7.2:

http://www.cs.cornell.edu/~ross/publications/tamewild/tamewi...

The Eclipse compiler only coincidentally rejects one of these programs due to this type inference problem. Other valid, unsound programs are accepted by it, and these programs are unsound due to the same basic principles the original (rejected one) used. The Unsound9.java example should still work.

The fact a compiler is rejecting a valid program (a bug in the eclipse compiler) is essentially orthogonal to the fact the program it rejects may show the type system is unsound (a bug in the language itself, so to speak).

Furthermore your parent post is still simply wrong. You say:

> Java's type system is _sound_ as it guarantees that you will never use a variable of one type as another incompatible type.

But this is literally the point of the paper. They cast Integer to String, you cannot get this through downcasting alone. It is unsound. They are able to write a function of type "a" to "b" for any types "a" and "b". It's almost the textbook definition of being able to break the type system, from the viewpoint of Java-the-language. This same "cast anything to anything else" is also considered a breaking of the type system in Haskell, too, and being able to write a similar "coerce" function is Very Bad.

In fact, it's very similar in theory to the way you might construct or leverage such a "bug" in Haskell. They are essentially constructing an argument to the compiler -- in the form of a type, which is how the compiler thinks -- that there is an equality between two arbitrary types A and B, so we can coerce between them. The compiler needs some evidence that this equality, this argument that A and B are the same, is legitimate. Otherwise, it rejects the argument, because it's not that gullible (it rejects the program as "this program failed to type check"). Because the argument to the compiler is made in the form of a type, the compiler considers "evidence" for that argument, to be a value of that type. So if your type makes the argument that "Integer and String are equal", then you need to represent that argument as a type, and create evidence for it by creating a value of that type. Then the compiler thinks your argument is legitimate. (This is a core central component to e.g. constructive mathematics/constructive computerized theorem proving)

`null` inhabits every type, so every type contains `null`. Even the type that says "String equals Integer" has `null` as a value. Thus, we can "prove" to the compiler that yes, we can turn an Integer into a String, we have the argument (the type) and the evidence (the value) to prove it! But the evidence is actually bogus.

I honestly suggest you give the original paper and the surrounding references a good reading. It all seems very sound and straightforward to me as an argument (with a bit of PLT background), and frankly I can't make heads or tails of most of your complaints in this thread, though if you're unfamiliar with the field, teasing out the subtle parts is probably a bit difficult.