Compiler wrongly identify value classes during desugaring (dual of #21918)

[hamzaremmal]

Compiler version

bed0e86

Minimized code

class AnyVal
class Foo extends AnyVal

Output

During typer, the desugaring assumes we are implementing a value class and generates the companion object for Foo

[[syntax trees at end of                     typer]] // t.scala
package <empty> {
  class AnyVal() extends Object() {}
  class Foo() extends AnyVal() {}
  final lazy module val Foo: Foo = new Foo()
  final module class Foo() extends AnyRef() { this: Foo.type =>}
}

Expectation

Should behave the same way as extending any other reference class:

[[syntax trees at end of                     typer]] // t.scala
package <empty> {
  class Anyval() extends Object() {}
  class Foo() extends Anyval() {}
}

Note

This is has the same root problem as #21918. In #21918, it was a case where isAnyVal wrongly unidentify value classes (false negative) while this issue is the case where the same check wrongly identify value classes (false positive).

scala3/compiler/src/dotty/tools/dotc/ast/Desugar.scala

Lines 597 to 601 in cc38962

	def isAnyVal(tree: Tree): Boolean = tree match {
	case Ident(tpnme.AnyVal) => true
	case Select(qual, tpnme.AnyVal) => isScala(qual)
	case _ => false
	}

[noti0na1]

I think a class extending AnyVal with no parameter should be illegal: https://docs.scala-lang.org/sips/value-classes.html

[hamzaremmal]

I think a class extending AnyVal with no parameter should be illegal: https://docs.scala-lang.org/sips/value-classes.html

It is illegal, the point of the issue is that desugaring wrongly identifies value classes and generates unnecessary extra code.