Use Scala2x flags for unpickled Scala 2 stdlib TASTy

Also introduce `Scala2Tasty` flag to differentiate between Scala 2
symbols that come from class files and Scala 2 symbols that come from
TASTy.

At this point, the only Scala 2 TASTy is the one from the standard
library. To know that a TASTy contains the definitions of the standard
library, we add the `Scala2StandardLibrary` attribute to the TASTy file.
We mark all unpickled classes from those TASTy files with `Scala2x | Scala2Tasty`.

compiler/src/dotty/tools/dotc/core/Flags.scala

@@ -308,8 +308,8 @@ object Flags {
    */
   val (_, StableRealizable @ _, _) = newFlags(24, "<stable>")
 
-  /** A case parameter accessor */
-  val (_, CaseAccessor @ _, _) = newFlags(25, "<caseaccessor>")
+  /** A case parameter accessor / an unpickled Scala 2 TASTy (only for Scala 2 stdlib) */
+  val (_, CaseAccessor @ _, Scala2Tasty @ _) = newFlags(25, "<caseaccessor>", "<scala-2-tasty>")
 
   /** A Scala 2x super accessor / an unpickled Scala 2.x class */
   val (SuperParamAliasOrScala2x @ _, SuperParamAlias @ _, Scala2x @ _) = newFlags(26, "<super-param-alias>", "<scala-2.x>")

compiler/src/dotty/tools/dotc/core/tasty/AttributeUnpickler.scala

@@ -3,7 +3,7 @@ package core.tasty
 
 import scala.language.unsafeNulls
 
-import dotty.tools.tasty.{TastyReader, TastyBuffer}
+import dotty.tools.tasty.{TastyFormat, TastyReader, TastyBuffer}
 
 import java.nio.charset.StandardCharsets
 
@@ -23,4 +23,6 @@ class AttributeUnpickler(reader: TastyReader):
     attributesBuilder.result()
   }
 
+  def scala2Stdlib: Boolean = attributes.contains(TastyFormat.Scala2StandardLibraryAttribute)
+
 end AttributeUnpickler

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -99,6 +99,9 @@ class TreeUnpickler(reader: TastyReader,
   /** Was unpickled class compiled with capture checks? */
   private var withCaptureChecks: Boolean = false
 
+  private val unpicklingScala2Lib =
+    attributeUnpicklerOpt.exists(_.scala2Stdlib)
+
   private def registerSym(addr: Addr, sym: Symbol) =
     symAtAddr(addr) = sym
 
@@ -610,7 +613,8 @@ class TreeUnpickler(reader: TastyReader,
       val rhsStart = currentAddr
       val rhsIsEmpty = nothingButMods(end)
       if (!rhsIsEmpty) skipTree()
-      val (givenFlags, annotFns, privateWithin) = readModifiers(end)
+      val (givenFlags0, annotFns, privateWithin) = readModifiers(end)
+      val givenFlags = if isClass && unpicklingScala2Lib then givenFlags0 | Scala2x | Scala2Tasty else givenFlags0
       pickling.println(i"creating symbol $name at $start with flags ${givenFlags.flagsString}, isAbsType = $isAbsType, $ttag")
       val flags = normalizeFlags(tag, givenFlags, name, isAbsType, rhsIsEmpty)
       def adjustIfModule(completer: LazyType) =

compiler/src/dotty/tools/dotc/transform/ExtensionMethods.scala

@@ -77,7 +77,7 @@ class ExtensionMethods extends MiniPhase with DenotTransformer with FullParamete
 
           // Create extension methods, except if the class comes from Scala 2
           // because it adds extension methods before pickling.
-          if (!(valueClass.is(Scala2x)))
+          if !valueClass.is(Scala2x, butNot = Scala2Tasty) then
             for (decl <- valueClass.classInfo.decls)
               if isMethodWithExtension(decl) then
                 enterInModuleClass(createExtensionMethod(decl, moduleClassSym.symbol))

compiler/src/dotty/tools/dotc/transform/Pickler.scala

@@ -16,6 +16,7 @@ import reporting.{ThrowingReporter, Profile, Message}
 import collection.mutable
 import util.concurrent.{Executor, Future}
 import compiletime.uninitialized
+import dotty.tools.tasty.TastyFormat.Scala2StandardLibraryAttribute
 
 object Pickler {
   val name: String = "pickler"
@@ -108,7 +109,9 @@ class Pickler extends Phase {
                 pickler, treePkl.buf.addrOfTree, treePkl.docString, tree,
                 scratch.commentBuffer)
 
-          val attributes = Nil // TODO add attributes here
+          val attributes =
+            if ctx.settings.Yscala2Stdlib.value then List(Scala2StandardLibraryAttribute)
+            else Nil
           AttributePickler.pickleAttributes(attributes, pickler, scratch.attributeBuffer)
 
           val pickled = pickler.assembleParts()

scala2-library-tasty-tests/src/Main.scala

@@ -17,6 +17,7 @@ object HelloWorld:
     testScala2ObjectParents()
     testScala2CaseClassUnderscoreMembers()
     testScalaNumberUnderlying()
+    testArrayOps()
     scala.collection.mutable.UnrolledBufferTest.test()
   }
 
@@ -68,3 +69,9 @@ object HelloWorld:
     val _: Object = MyNumber2(BigInt(1)).underlying
     val _: Object = (MyNumber2(BigInt(1)): ScalaNumber).underlying
   }
+
+  def testArrayOps() = {
+    new collection.ArrayOps[String](Array[String]("foo")).exists(x => true)
+  }
+
+end HelloWorld

tasty/src/dotty/tools/tasty/TastyFormat.scala

@@ -366,6 +366,8 @@ object TastyFormat {
   final val CommentsSection = "Comments"
   final val AttributesSection = "Attributes"
 
+  final val Scala2StandardLibraryAttribute = "Scala2StandardLibrary"
+
   /** Tags used to serialize names, should update [[TastyFormat$.nameTagToString]] if a new constant is added */
   class NameTags {
     final val UTF8 = 1               // A simple name in UTF8 encoding.