Value classes' extractable methods don't follow the pattern in the SIP

[hamzaremmal]

Compiler version

590691b

Minimized code

class Bar(val self: Short) extends AnyVal {
    private def foo = ???
}

Output

Scala 3

[[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()
        ()
      }
    override def hashCode(): Int = Bar.hashCode$extension(this.self())
    override def equals(x$0: Object): Boolean =
      Bar.equals$extension(this.self(), x$0)
    private val self: Short
    def self(): Short = this.self
    private def foo(): Nothing = Bar.Bar$$$foo$extension(this.self())
  }
  @SourceFile("playground.scala") final module class Bar extends Object {
    def <init>(): Unit =
      {
        super()
        ()
      }
    private def writeReplace(): Object =
      new scala.runtime.ModuleSerializationProxy(classOf[Bar])
    final def hashCode$extension($this: Short): Int =
      Short.box($this).hashCode()
    final def equals$extension($this: Short, x$0: Object): Boolean =
      matchResult1[Boolean]: 
        {
          case val x1: Object = x$0
          if x1.isInstanceOf[Bar] then
            {
              case val x$0: Short =
                if x1.eq(null) then Short.unbox(null) else 
                  x1.asInstanceOf[Bar].self()
              return[matchResult1] $this.==(x$0)
            }
           else ()
          return[matchResult1] false
        }
    final def Bar$$$foo$extension($this: Short): Nothing = ???()
  }
  final lazy module val Bar: Bar = new Bar()
}

Scala 2

[[syntax trees at end of                       jvm]] // playground.scala
package <empty> {
  final class Bar extends Object {
    <paramaccessor> private[this] val self: Short = _;
    <stable> <accessor> <paramaccessor> def self(): Short = Bar.this.self;
    private def foo(): Nothing = Bar.foo$extension(Bar.this.self());
    override <synthetic> def hashCode(): Int = Bar.hashCode$extension(Bar.this.self());
    override <synthetic> def equals(x$1: Object): Boolean = Bar.equals$extension(Bar.this.self(), x$1);
    def <init>(self: Short): Bar = {
      Bar.this.self = self;
      Bar.super.<init>();
      ()
    }
  };
  <synthetic> object Bar extends Object {
    final def foo$extension($this: Short): Nothing = scala.Predef.???();
    final <synthetic> def hashCode$extension($this: Short): Int = java.lang.Short.hashCode($this);
    final <synthetic> def equals$extension($this: Short, x$1: Object): Boolean = {
  case <synthetic> val x1: Object = x$1;
  case5(){
    if (x1.$isInstanceOf[Bar]())
      matchEnd4(true)
    else
      case6()
  };
  case6(){
    matchEnd4(false)
  };
  matchEnd4(x: Boolean){
    x
  }
}.&&({
      <synthetic> val Bar$1: Short = x$1.$asInstanceOf[Bar]().self();
      $this.==(Bar$1)
    });
    def <init>(): Bar.type = {
      Bar.super.<init>();
      ()
    }
  }
}

Expectation

Dotty should follow SIP-15 and Scala 2 and generate a method called foo$extension not Bar$$$foo$extension

[hamzaremmal]

For now, I'm not considering this issue as a blocker for #22480 because value classes's extension methods are implementation details, and those methods are private so the resolution will always have the correct method to call.
Nevertheless, raising the issue to clear the specification once and for all.