Inference variable has incompatible bounds. Java 8 Compiler Regression?

Question

The following program compiles in Java 7 and in Eclipse Mars RC2 for Java 8:

import java.util.List;

public class Test {

    static final void a(Class

        

Answer 1

Disclaimer - I don't know enough about the subject, and the following is an informal reasoning of mine to try to justify javac's behavior.

---

We can reduce the problem to

> void a(Class type) throws Exception
{
    X instance = type.newInstance();
    b(instance);  // error
}

 List b(List list) { ... }

To infer T, we have constraints

      X <: List
      X <: List

Essentially, this is unsolvable. For example, no T exists if X=List.

Not sure how Java7 infers this case. But javac8 (and IntelliJ) behaves "reasonably", I'd say.

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Now, how come this workaround works?

    List instance = type.newInstance();
    b(instance);  // ok!

It works due to wildcard capture, which introduces more type info, "narrowing" the type of instance

    instance is List  =>  exist W, where instance is List  =>  T=W

Unfortunately, this is not done when instance is X, thus there is less type info to work with.

Conceivably, the language could be "improved" to do wildcard capture for X too:

    instance is X, X is List  =>  exist W, where instance is List