Make sure that the Scala 2 library CC compiles on the stable branch (#19973)

Author: Linyxus

project/Build.scala

@@ -1066,6 +1066,7 @@ object Build {
       Compile / doc / scalacOptions += "-Ydocument-synthetic-types",
       scalacOptions += "-Ycompile-scala2-library",
       scalacOptions += "-Yscala2Unpickler:never",
+      scalacOptions += "-Yno-experimental",
       scalacOptions -= "-Xfatal-warnings",
       Compile / compile / logLevel := Level.Error,
       ivyConfigurations += SourceDeps.hide,

scala2-library-cc/src/scala/collection/generic/IsSeq.scala

@@ -58,17 +58,17 @@ object IsSeq {
     * The helper method [[seqViewIsSeq_]] is added to make the binary compatible.
     */
   @annotation.targetName("seqViewIsSeq")
-  @annotation.publicInBinary
-  private[IsSeq] def seqViewIsSeq_[CC0[X] <: SeqView[X], A0]: IsSeq[CC0[A0]] { type A = A0; type C = View[A0] } = ???
+  // @annotation.publicInBinary private[IsSeq] // FIXME: enable when publicInBinary becomes stable
+  def seqViewIsSeq_[CC0[X] <: SeqView[X], A0]: IsSeq[CC0[A0]] { type A = A0; type C = View[A0] } = ???
   implicit inline def seqViewIsSeq[CC0[X] <: SeqView[X], A0]: IsIterable[CC0[A0]] { type A = A0; type C = View[A0] } = seqViewIsSeq_[CC0, A0].asInstanceOf
 
   /** !!! Under cc, views are not Seqs and can't use SeqOps.
     * Therefore, [[stringViewIsSeq]] now returns an [[IsIterable]].
     * The helper method [[stringViewIsSeq__]] is added to make the binary compatible.
     */
   @annotation.targetName("stringViewIsSeq")
-  @annotation.publicInBinary
-  private[IsSeq] val stringViewIsSeq_ : IsSeq[StringView] { type A = Char; type C = View[Char] } = ???
+  // @annotation.publicInBinary private[IsSeq] // FIXME: enable when publicInBinary becomes stable
+  val stringViewIsSeq_ : IsSeq[StringView] { type A = Char; type C = View[Char] } = ???
   inline implicit def stringViewIsSeq: IsIterable[StringView] { type A = Char; type C = View[Char] } = stringViewIsSeq_.asInstanceOf
 
   implicit val stringIsSeq: IsSeq[String] { type A = Char; type C = String } =