Fix pkg obj prefix of opaque tp ext meth

[dwijnand]

TypeOps.makePackageObjPrefixExplicit is a part of accessibleType,
which is called on the result of findRef in typedIdent. But in
tryExtension it's not. We could fix it in the usage of the results in
tryExtension, but I thought perhaps we could fix it for all call
sites, by handling it within findRef.

Fixes #18097

[odersky]

I think this is fine but could profit from a refactoring.

• Move makePackageObjPrefixExplciit from TypeOps to TypeUtils and make it an extension method on Type
• Drop the extension method in findRef.

[WojciechMazur]

@dwijnand
2 projects in OpenCB started to fail after including these changes.
It seems like previously opaque type rhs type could have been interpreted as opaque type.
I think it now works as expected, but please confirm it based on the following reproducers. Probably it would be useful to include these cases in the compiler tests to prevent restoring old behaviour in the future.

zainab-ali/aquascape

Build logs

abstract class Ref[F[_], A]

trait FunctorOps[F[_], A]:
  def map[B](f: A => B): F[B] = ???

type Branch = String
type Label = String
opaque type Stack[F[_]] = Ref[F, Map[Branch, List[Label]]]
extension [F[_]](stack: Stack[F])
  def bracketF[A](branch: Branch, child: Label)(fa: F[A]): F[A] = ???

trait Pen[F[_], E]:
  def bracket[A](branch: Branch, child: Label)(fa: F[A]): F[A]

object Pen {
  def apply[F[_], E]: F[Pen[F, E]] =
    null
    .asInstanceOf[FunctorOps[F, Ref[F, Map[Branch, List[Label]]]]] // fails
    // .asInstanceOf[FunctorOps[F, Stack[F]]] // works
    .map { stack =>
      new Pen[F, E] {
        def bracket[A](branch: Branch, child: Label)(fa: F[A]): F[A] = stack.bracketF(branch, child)(fa)
      }
    }
}

yields

[error] ./test.scala:22:72
[error] value bracketF is not a member of Ref[F, Map[Branch, List[Label]]].
[error] An extension method was tried, but could not be fully constructed:
[error] 
[error]     bracketF[F](stack)
[error] 
[error]     failed with:
[error] 
[error]         Found:    (stack : Ref[F, Map[Branch, List[Label]]])
[error]         Required: Stack[F²]
[error]         
[error]         where:    F  is a type in method apply with bounds <: [_] =>> Any
[error]                   F² is a type variable with constraint <: [_²] =>> Any
[error]         
[error] 
[error] where:    F is a type in method apply with bounds <: [_] =>> Any
[error]         def bracket[A](branch: Branch, child: Label)(fa: F[A]): F[A] = stack.bracketF(branch, child)(fa)
[error]      

bilal-fazlani/zio-maelstrom

Build logs

// defs.scala
opaque type NodeId = String
opaque type MessageSource = NodeId
extension (s: MessageSource)
  def nodeId: NodeId = s

// usage.scala
private def suspend(remote: NodeId) = remote.nodeId
class Logic:
  private def suspend(remote: NodeId) = remote.nodeId

yields

-- [E008] Not Found Error: /Users/wmazur/projects/scala/sandbox/.zio-mealstrom/usage.scala:3:47 -----------------------------------------------------------------------------------------------
3 |  private def suspend(remote: NodeId) = remote.nodeId
  |                                        ^^^^^^^^^^^^^
  |                                        value nodeId is not a member of NodeId.
  |                                        An extension method was tried, but could not be fully constructed:
  |
  |                                            nodeId(remote)
  |
  |                                            failed with:
  |
  |                                                Found:    (remote : NodeId)
  |                                                Required: MessageSource
-- [E008] Not Found Error: /Users/wmazur/projects/scala/sandbox/.zio-mealstrom/usage.scala:1:45 -----------------------------------------------------------------------------------------------
1 |private def suspend(remote: NodeId) = remote.nodeId
  |                                      ^^^^^^^^^^^^^
  |                                      value nodeId is not a member of NodeId.
  |                                      An extension method was tried, but could not be fully constructed:
  |
  |                                          nodeId(remote)
  |
  |                                          failed with:
  |
  |                                              Found:    (remote : NodeId)
  |                                              Required: MessageSource

[dwijnand]

Yes, that all looks correct. I'll PR the cases, thank you!

[WojciechMazur]

FYI @zainab-ali @bilal-fazlani