[External] : Re: Making Object abstract

[John Rose]

On Jun 17, 2021, at 4:40 AM, Remi Forax <forax at univ-mlv.fr<mailto:forax at univ-mlv.fr>> wrote:

As a stretch move, I think we can even retro-upgrade
the type checking of Objects::newIdentity with type
variable trickery, when IdentityObject becomes real.

Please see:

https://bugs.openjdk.java.net/secure/attachment/95170/Foo.java
https://bugs.openjdk.java.net/browse/JDK-8268919


I wonder if a simple way to avoid to allow any Ts if to allow to specify an intersection type as parameter type and/or return type of methods (we have 'var' inside the body)
so instead of

 public static <T extends Object & IdentityObject> T newIdentity() {

we can write

 public static Object & IdentityObject newIdentity() {

This requires a grammar change but it's exactly the type we want.

Yes.  I was trying to avoid that.  My attempt (see above)
is wrong because (as Dan points out) it can infer T to
be String, which is false.  The type variable T would
need lower bound of Identity, as well as an upper
bound (and erasure) of Object.  But, as with return
type intersections, there is no syntax to express
that, apparently.  Only wildcards can have
lower bounds, right?  And you can’t have a
(lower-bounded) wildcard for a return type:

public static List<? super IdentityObject> newIdentityList(); //OK
public static ? super IdentityObject newIdentity(); //not OK



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