reproduce dotty Issue 16247 => open #16408

For details on the problem see Issue 16247: Should Java Interfaces be Matchable?. That issue was closed as the bug was not reproducible. The history of this github repository traces my attempts at duplicating it. This was successful with commit e26983bec0d1a509cab97ac44f78f45935f4a980 and led to reopening the issue as Problem with Traits not being Matchable #16408

An intial attempt seemed to reproduce the problem. but as per 26 Oct, discussion on Discord it turns out this seems to have been tied to my OS or java installation...

Currently to run the code make a clean install of java and scala.

I try to use minimal commands and limit what jdk and java compiler I am using

export JAVA_HOME=/Library/Java/JavaVirtualMachines/temurin-17.jdk/Contents/Home
export SCALA_HOME=/usr/local/scala/scala3-3.2.1/

To compile the code look at the two scripts in bin directory.

The RDF code based on Scala Classes works

➤  sh bin/compileScalaPure
Success(https://bblfish.net/#i)
folded=<https://bblfish.net/#i>
matched should be uriisURI class_based.ClassTypes$AFactory$$anon$2@358ee631with authority Success(bblfish.net)

The code relying on Java interfaces does not.

➤  sh bin/compileJava
-- Error: scala/RDF_Interface.scala:18:8 ---------------------------------------
18 |  given rops: ROps[R] with
   |        ^
   |object creation impossible, since protected def auth(uri: generic.RDF.URI[R]): util.Try[String] in trait ROps in package generic is not defined
   |(Note that
   | parameter generic.RDF.URI[R] in protected def auth(uri: generic.RDF.URI[R]): util.Try[String] in trait ROps in package generic does not match
   | parameter testorg.Uri & (testorg.TstNode & (testorg.TstNode & Matchable)) & (testorg.Uri &
   |
   |(testorg.TstNode & Matchable)) in protected override def auth
   |  (uri: testorg.Uri & (testorg.TstNode & (testorg.TstNode & Matchable)) & (
   |    testorg.Uri
   |   & (testorg.TstNode & Matchable))): util.Try[String] in object rops in object IRDF
   | )
-- [E038] Declaration Error: scala/RDF_Interface.scala:25:27 -------------------
25 |    override protected def auth(uri: RDF.URI[R]): Try[String] =
   |                           ^
   |   method auth has a different signature than the overridden declaration
   |----------------------------------------------------------------------------
   | Explanation (enabled by `-explain`)
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   | There must be a non-final field or method with the name auth and the
   | same parameter list in a super class of object rops to override it.
   |
   |   protected override def auth
   |   (uri: testorg.Uri & (testorg.TstNode & (testorg.TstNode & Matchable)) & (
   |     testorg.Uri
   |    & (testorg.TstNode & Matchable))): util.Try[String]
   |
   | The super classes of object rops contain the following members
   | named auth:
   |   protected def auth(uri: generic.RDF.URI[interf_based.IRDF.R]): util.Try[String]
    ----------------------------------------------------------------------------
2 errors found

Indeed the code relying on Scala traits does not compile either. Here we use scala-cli as we do in the github actions that are run whenever a commit is sent to this repository. See workflows reports.

scala-cli --scala 3.2.1 scala/RDF_Traits.scala scala/RDF_UsingScalaTrait.scala scala/RDF.scala

Notes

The problem was found in commit e26983bec0d1a509cab97ac44f78f45935f4a980 after adding a simple method auth to the ROps trait.

 protected def auth(uri: RDF.URI[R]): Try[String]

The next commit after that was to clean up the code into three sections:

• generic RDF -- all the code that does not know about implementations
• class-based -- the implementation that works using scala classes
• interface-based -- the implementation works using interfaces (Java ones in particular)

using a side by side diff tool such as opendiff on macos, will help show that the difference between RDF_Class and RDF_Interface is essentially just that RDF_Class needs to supply its own classes and factory.

cd scala
opendiff RDF_Class.scala  RDF_Interface.scala