extension method seen on first compilation, but not second in the repl

[johnynek]

minimized code

// unfortunately       
// opaque type Fix[F[_]] = F[Fix[F]]
// won't work (no recursion in opaque type), but this implementation is safe, but scary due to asInstanceOf
object FixImpl {
  type Fix[F[_]]

  inline def fix[F[_]](f: F[Fix[F]]): Fix[F] = f.asInstanceOf[Fix[F]]
  inline def unfix[F[_]](f: Fix[F]): F[Fix[F]] = f.asInstanceOf[F[Fix[F]]]
}

import FixImpl._

type OrNull[A <: AnyRef] = A | Null
// List[A] = null | (A, null) | (A, (A, null)) | (A, (A, (A, null))) | ...
opaque type List[A] = Fix[[X] =>> OrNull[(A, X)]]

object List {
  def empty[A]: List[A] = fix(null)
}

given ListOps {
  def (head: A) ::[A](self: List[A]): List[A] =
    fix((head, self))
} 

In the repl, I can use :: the first time, but not the second time:

scala> List.empty[Int]
val res0: List[Int] = null

scala> 1 :: res0
val res1: List[Int] = (1,null)

scala> 1 :: res0
1 |1 :: res0
  |  ^^^^^^^
  |  value :: is not a member of List[Int] - did you mean res0.==?

expectation

Notice, the first time I use :: it works, the second time with the exact same syntax, it fails.

seems possibly related to #7181

[som-snytt]

As top-level in a named or empty package, if List is opaque:

scala> { import i7182._
     | 42 :: res0
     | }
2 |42 :: res0
  |   ^^^^^^^
  |   value :: is not a member of i7182.List[Int].
  |   An extension method was tried, but could not be fully constructed:
  |
  |       i7182.::[A](res0)

That is after deleting the disallowed inline and writing :: as extension.

I remember that top level opaque means opaque in the package object, but it's not obvious why compilation fails.

It works if not opaque or not top level:

scala> { import i7182.repro._
     | List.empty[Int]
     | }
val res0: i7182.repro.List[Int] = null

scala> { import i7182.repro._
     | 42 :: res0
     | }
val res1: i7182.repro.List[Int] = (42,null)

scala> { import i7182.repro._
     | 42 :: res0
     | }
val res2: i7182.repro.List[Int] = (42,null)

Actually it occurs to me that it's easier to rename the type for purposes of this experiment:

scala> import i7182.repro._

scala> Lost.empty[Int]
val res0: i7182.repro.Lost[Int] = null

scala> 42 :: res0
val res1: i7182.repro.Lost[Int] = (42,null)

scala> 42 :: res0
val res2: i7182.repro.Lost[Int] = (42,null)

[Kordyjan]

I've tried to reproduce this issue during the Spree, and have the same conclusions as @som-snytt above. Putting the example in an object body and removing the inlines, fixes it. The bug has been probably fixed along the way, but should be checked again as soon as #12748 #9879 is fixed.

[dwijnand]

I renamed List to LList and defined the extension method as such:

extension [A](head: A)
  def ::(self: LList[A]): LList[A] =
    fix((head, self))

and it's broken in 3.0.1 (not even the first call succeeds), but it seems to be fixed in 3.0.2-RC1:

$ scalac -3.0.2-RC1 i7182.scala -d out/
$ scala -3.0.2-RC1 -cp out
scala> LList.empty[Int]
val res0: LList[Int] = null

scala> 1 :: res0
val res1: LList[Int] = (1,null)

scala> 1 :: res0
val res2: LList[Int] = (1,null)