Impossible to extends "Ordered" when "-Yexplicit-nulls" flag used

[flomebul]

Compiler version

scala version 3.0.0-M3

Minimized code

object Issue11341 {
  class ClsA(i: Int) extends Ordered[ClsA] {
    def compare(that: ClsA): Int = ???

    // Members declared in java.lang.Comparable
    override def compareTo(that: ClsA | UncheckedNull): Int = ???
  } 

  class ClsB(i: Int) extends Ordered[ClsB] {
    def compare(that: ClsB): Int = ???

    // Members declared in java.lang.Comparable
    override def compareTo(that: ClsB): Int = ???
  } 

  class ClsC(i: Int) extends Ordered[ClsC] {
    def compare(that: ClsC): Int = ???
  } 
}

Output

def compareTo(that: asuivre.Issue11341.ClsA | UncheckedNull): Int
error overriding method compareTo in trait Ordered of type (that: asuivre.Issue11341.ClsA): Int;
  method compareTo of type (that: asuivre.Issue11341.ClsA | UncheckedNull): Int has incompatible type

class ClsB: asuivre.Issue11341.ClsB
class ClsB needs to be abstract, since def compareTo(x$0: T | UncheckedNull): Int is not defined 
(Note that T | UncheckedNull does not match asuivre.Issue11341.ClsB)

def compareTo(that: asuivre.Issue11341.ClsB): Int
error overriding method compareTo in trait Comparable of type (x$0: asuivre.Issue11341.ClsB | UncheckedNull): Int;
  method compareTo of type (that: asuivre.Issue11341.ClsB): Int has incompatible type

class ClsC: asuivre.Issue11341.ClsC
class ClsC needs to be abstract, since def compareTo(x$0: T | UncheckedNull): Int is not defined 
(Note that T | UncheckedNull does not match asuivre.Issue11341.ClsC)

Expectation

I suppose that the case "ClsC" is the right one, as it is the case without the "-Yexplicit-nulls" flag.

[noti0na1]

I tested this example using the new explicit nulls PR #9884 (We no longer have UncheckedNull, so it need to be repleced by Null).

The output is:

    |  override def compareTo(that: ClsA | Null): Int = ???
    |               ^
    |error overriding method compareTo in trait Ordered of type (that: ClsA): Int;
    |  method compareTo of type (that: ClsA | Null): Int has incompatible type

I think this is expected in the current design. In the PR, we have a relaxed overriding rule when the overrided member is defined in Java.

However in this case, Ordered is a scala trait, and it overrides the compareTo function. Therefore, compareTo becomes a scala function and the special rule doesn't apply. The only way to override compareTo here is override def compareTo(that: ClsA): Int.

In theory, we may compare an object to a null pointer, but the current standard lib is not compiled using explicit nulls, we are unable to add | Null to the param type.

A workaround is to cast that: ClsA to ClsA | Null and then compare it to null.

[olhotak]

Do ClsB and ClsC compile with #9884?

I think that is the desired behaviour in this case, that ClsB and ClsC compile but ClsA doesn't.

[noti0na1]

@olhotak Yes, ClsB and ClsC can be compiled successfully.

[flomebul]

If ClsC compile, it is exactly what is expected. Thanks