Dotty allow overriding equals and hashCode for Value Classes

[hamzaremmal]

Compiler version

590691b

Minimized code

class Bar(val self: Short) extends AnyVal:
    override def equals(that: Any): Boolean = ???
    override def hashCode: Int = ???

Output

[[syntax trees at end of                  genBCode]] // playground.scala
package <empty> {
  @SourceFile("playground.scala") final class Bar extends Object {
    def <init>(self: Short): Unit =
      {
        this.self = self
        super()
        ()
      }
    private val self: Short
    def self(): Short = this.self
    override def equals(that: Object): Boolean =
      Bar.equals$extension(this.self(), that)
    override def hashCode(): Int = Bar.hashCode$extension(this.self())
  }
  @SourceFile("playground.scala") final module class Bar extends Object {
    def <init>(): Unit =
      {
        super()
        ()
      }
    private def writeReplace(): Object =
      new scala.runtime.ModuleSerializationProxy(classOf[Bar])
    final def equals$extension($this: Short, that: Object): Boolean = ???()
    final def hashCode$extension($this: Short): Int = ???()
  }
  final lazy module val Bar: Bar = new Bar()
}

Compiles.

Expectation

Dotty should prohibit overriding equals and hashCode per the Value Classes's specification; SIP-15:

A value class C must satisfy the following criteria:
...
5. C may not define concrete equals or hashCode methods
...

[soronpo]

Other than it being out of spec, what is the issue? Maybe it's better to change the spec since it has been like this from the start.

[hamzaremmal]

@soronpo If the spec changes, we will have to implement the old behaviour anyway for #22480 (behind -Ycompile-scala2-library).

EDIT: Maybe this coment is more relevant for #22493 that flag the change of semantics

[hamzaremmal]

Actually, for #22480, since we can define both these methods in source, we can just write them manually to preserve the semantics of those specific classes. So the question is open: Should the implementation change or the spec ?

[soronpo]

I'm using this capability for utilizing the standard collection with my own custom comparison (e.g., grouping lists according to a different criteria than the default equality). I will admit that I'm not sure if indeed the value class saves on boxing/unboxing values for my use-case.

[mbovel]

This issue was picked for the Scala Issue Spree of tomorrow, Monday, February 3rd. @hamzaremmal, @julian-a-avar-c and @nmcb will be working on it.

[sjrd]

I have a vague recollection that this change was intentional. It's always been frustrating that we couldn't override equals and hashCode for value classes in Scala 2. Since this language restriction is not necessary for correctness, and has no impact on the implementation strategy, I believe we chose not to restrict in Scala 3.

At this point I think we should change the spec instead.

[nmcb]

I'm not sure if indeed the value class saves on boxing/unboxing values for my use-case.

@soronpo can you link to your use-case ?

[mbovel]

@hamzaremmal: if this is in the end should be a specification change, should we try to find an alternative issue for today's Spree?

[hamzaremmal]

@mbovel Yes please

[mbovel]

@mbovel Yes please

I suggest that you tackle #22232 instead.

[sjrd]

@soronpo can you link to your use-case ?

Here would be my use case ("would" because this code still cross-compiles for Scala 2.12, so ...):
https://github.com/scala-js/scala-js/blob/48ddd575a43eae47d6e96138e0e1749f0ea50b3c/ir/shared/src/main/scala/org/scalajs/ir/UTF8String.scala#L25-L29

@note
equals() and hashCode(), along with == and ##, are just as
broken for UTF8String as for Arrays. Use the methods in the
companion object instead. This is unavoidable because we cannot override
equals nor hashCode in an AnyVal.

https://github.com/scala-js/scala-js/blob/48ddd575a43eae47d6e96138e0e1749f0ea50b3c/ir/shared/src/main/scala/org/scalajs/ir/UTF8String.scala#L97-L101

  def equals(x: UTF8String, y: UTF8String): Boolean =
    java.util.Arrays.equals(x.bytes, y.bytes)

  def hashCode(x: UTF8String): Int =
    scala.util.hashing.MurmurHash3.bytesHash(x.bytes)

[hamzaremmal]

In today's Core Meeting, we have decided to modify the specification of Value Classes to allow the definition of equals and hashCode. The decision is motivated by the fact that we don't see any reason why we should impose this limitation.
I will leave this ticket open for now until we change the specification.