Indirectly extending a trait with parameters works (unexpectedly)

[alvinj]

Compiler version

Scala 3.0-M3

Minimized code

In a Gitter discussion that starts here, I noted that in following this documentation page I created this example, which works as desired:

trait Pet(val name: String)
trait FeatheredPet extends Pet

// the docs say to do this
class Bird(override val name: String) extends Pet(name), FeatheredPet:
   override def toString = s"bird name: $name"

Then out of curiosity I also tried this, and saw that it works as well:

// this also works
class Bird(override val name: String) extends FeatheredPet:
   override def toString = s"bird name: $name"

On Gitter, @smarter noted that this second behavior might be an issue, so I’m reporting it here.

[smarter]

I put the "needs spec" label but I think the spec is fine (SIP-25 says "If a class C implements a parameterized trait T, and its superclass does not, then T must appear as a parent trait of C with arguments.") and it's the implementation which is missing a check, /cc @odersky.

[som-snytt]

TIL I don't know how trait parameters are supposed to work.

[success] Total time: 170 s (02:50), completed Jan 25, 2021, 6:39:32 PM
Starting scala3 REPL...
scala> trait Pet(val name: String, rest: String):
     |   def f(suffix: String) = s"$name$suffix$rest"
     |
// defined trait Pet

scala> class Birdie(override val name: String) extends Pet("huh", "more")
// defined class Birdie

scala> Birdie("Polly").f("Cracker")
val res0: String = PollyCrackerhuh     // expected s/huh/more

Without the trait val and override, it's correctly

scala> Birdie("Polly").f("Cracker")
val res1: String = huhCrackermore

The unspecified behavior of not requiring args when val params are overridden (and params aren't used in initialization) seems OK to me:

scala> trait Pet(val name: String, val rest: String):
     |   def f(suffix: String) = s"$name$suffix$rest"
     |
// defined trait Pet

scala> trait X extends Pet
// defined trait X

scala> class Birdie(override val name: String) extends X
1 |class Birdie(override val name: String) extends X
  |      ^
  |      parameterized trait Pet is indirectly implemented,
  |      needs to be implemented directly so that arguments can be passed

scala> class Birdie(override val name: String, override val rest: String) extends X
// defined class Birdie

scala> Birdie("Polly", "Parrot").f("Cracker")
val res2: String = PollyCrackerParrot

[som-snytt]

I would naively expect the same behavior:

scala> class C(val s: String) { def f = s * 2 ; println(s) }
// defined class C

scala> class D(override val s: String) extends C("bye")
// defined class D

scala> D("hello").f
bye
val res10: String = hellohello

scala> trait C(val s: String) { def f = s * 2 ; println(s) }
// defined trait C

scala> class D(override val s: String) extends C("bye")
// defined class D

scala> D("hello").f
hello
val res11: String = hellohello

I have to re-read what the docs say about initialization.

I think the original "One-Question FAQ" from Scala 2, which had one question about initialization, will still have only question, but now it will have multiple answers.

[som-snytt]

Actually #9460