Reinterpret xyz$access$i when unpickling from Scala 2

Fixes scala#18884

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -168,6 +168,13 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
   /** A mapping from method types to the parameters used in constructing them */
   private val paramsOfMethodType = new java.util.IdentityHashMap[MethodType, List[Symbol]]
 
+  /** Set of var param accessors that need to become case accessors */
+  private val paramAccessorNeedsCaseAccessor = mutable.Map.empty[ClassSymbol, mutable.Set[Name]]
+
+  /** If a var param accessors that need to become case accessors */
+  private def paramAccessorNeedsCaseAccessor(owner: ClassSymbol, name: Name): Boolean =
+    paramAccessorNeedsCaseAccessor.get(owner).exists(_.contains(name))
+
   protected def errorBadSignature(msg: String, original: Option[RuntimeException] = None)(using Context): Nothing = {
     val ex = new BadSignature(
       i"""error reading Scala signature of $classRoot from $source:
@@ -462,6 +469,14 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
       flags &~= Method | Accessor
       if !flags.is(StableRealizable) then flags |= Mutable
 
+    // Skip case accessor `<xyz>$access$<idx>` and keep track of their name to make `<xyz>` the case accessor
+    if flags.is(CaseAccessor) && name.toString().contains("$access$") then
+      val accessorName = name.toString().split('$').head.toTermName
+      paramAccessorNeedsCaseAccessor
+        .getOrElseUpdate(owner.asClass, mutable.Set.empty)
+        .+=(accessorName)
+      return NoSymbol // skip this member
+
     name = name.adjustIfModuleClass(flags)
     if (flags.is(Method))
       name =
@@ -632,6 +647,11 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
                 // This is the `def` of an accessor that needs to be transformed into
                 // a `val`/`var`. Note that the `Method | Accessor` flags were already
                 // striped away in `readDisambiguatedSymbol`.
+                //
+                // If there was a `<xyz>$access$<idx>` case accessor, we also need to
+                // make this accessor a case accessor.
+                if paramAccessorNeedsCaseAccessor(denot.owner.asClass, denot.name) then
+                  denot.setFlag(CaseAccessor)
                 resultType
               case tp1 => tp1
             denot.info =

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -1058,7 +1058,7 @@ class DottyBytecodeTests extends DottyBytecodeTest {
         TypeOp(CHECKCAST, "scala/collection/immutable/$colon$colon"),
         VarOp(ASTORE, 3),
         VarOp(ALOAD, 3),
-        Invoke(INVOKEVIRTUAL, "scala/collection/immutable/$colon$colon", "next$access$1", "()Lscala/collection/immutable/List;", false),
+        Invoke(INVOKEVIRTUAL, "scala/collection/immutable/$colon$colon", "next", "()Lscala/collection/immutable/List;", false),
         VarOp(ASTORE, 4),
         VarOp(ALOAD, 3),
         Invoke(INVOKEVIRTUAL, "scala/collection/immutable/$colon$colon", "head", "()Ljava/lang/Object;", false),
@@ -1112,7 +1112,7 @@ class DottyBytecodeTests extends DottyBytecodeTest {
         Invoke(INVOKESTATIC, "scala/runtime/BoxesRunTime", "unboxToInt", "(Ljava/lang/Object;)I", false),
         VarOp(ISTORE, 4),
         VarOp(ALOAD, 3),
-        Invoke(INVOKEVIRTUAL, "scala/collection/immutable/$colon$colon", "next$access$1", "()Lscala/collection/immutable/List;", false),
+        Invoke(INVOKEVIRTUAL, "scala/collection/immutable/$colon$colon", "next", "()Lscala/collection/immutable/List;", false),
         VarOp(ASTORE, 5),
         Op(ICONST_1),
         VarOp(ILOAD, 4),

compiler/test/dotty/tools/vulpix/ParallelTesting.scala

@@ -616,7 +616,7 @@ trait ParallelTesting extends RunnerOrchestration { self =>
 
       def scala2Command(): Array[String] = {
         assert(!flags.options.contains("-scalajs"),
-          "Compilation tests with Scala.js on Scala 2 are not supported.\nThis test can be blacklisted in compiler/test/dotc/*-scala-js.blacklist")
+          "Compilation tests with Scala.js on Scala 2 are not supported.\nThis test can be skipped using the `// scalajs: --skip` tag")
         val stdlibClasspath = artifactClasspath("org.scala-lang", "scala-library")
         val scalacClasspath = artifactClasspath("org.scala-lang", "scala-compiler")
         val flagsArgs = flags

tests/neg/i18884.scala

@@ -0,0 +1,2 @@
+def test(xs: ::[Int]): List[Int] =
+  xs.next$access$1 // error

tests/run/i18884.check

@@ -0,0 +1,4 @@
+Foo1(1)
+Foo2(2, 3)
+Foo3(4, 5)
+Foo4(6, 7)

tests/run/i18884/A_1_c2.13.12.scala

@@ -0,0 +1,10 @@
+// scalajs: --skip
+
+package lib
+
+case class Foo1(private[lib] var x: Int) {}
+case class Foo2(private[lib] var x: Int, private[lib] var y: Int)
+case class Foo3(private[lib] var x: Int, var y: Int)
+case class Foo4(var x: Int, private[lib] var y: Int) {
+  val z: Int = x
+}

tests/run/i18884/B_2.scala

@@ -0,0 +1,14 @@
+import lib.*
+
+@main def Test: Unit =
+  test(new Foo1(1))
+  test(new Foo2(2, 3))
+  test(new Foo3(4, 5))
+  test(new Foo4(6, 7))
+
+def test(any: Any): Unit =
+  any match
+    case Foo1(x) => println(s"Foo1($x)")
+    case Foo2(x, y) => println(s"Foo2($x, $y)")
+    case Foo3(x, y) => println(s"Foo3($x, $y)")
+    case Foo4(x, y) => println(s"Foo4($x, $y)")

tests/run/typeclass-derivation3.check

@@ -9,6 +9,6 @@ Cons(hd = Cons(hd = 11, tl = Cons(hd = 22, tl = Cons(hd = 33, tl = Nil))), tl =
 Cons(hd = Left(x = 1), tl = Cons(hd = Right(x = Pair(x = 2, y = 3)), tl = Nil))
 Cons(hd = Left(x = 1), tl = Cons(hd = Right(x = Pair(x = 2, y = 3)), tl = Nil))
 true
-::(head = 1, next$access$1 = ::(head = 2, next$access$1 = ::(head = 3, next$access$1 = Nil())))
-::(head = ::(head = 1, next$access$1 = Nil()), next$access$1 = ::(head = ::(head = 2, next$access$1 = ::(head = 3, next$access$1 = Nil())), next$access$1 = Nil()))
-::(head = Nil(), next$access$1 = ::(head = ::(head = 1, next$access$1 = Nil()), next$access$1 = ::(head = ::(head = 2, next$access$1 = ::(head = 3, next$access$1 = Nil())), next$access$1 = Nil())))
+::(head = 1, next = ::(head = 2, next = ::(head = 3, next = Nil())))
+::(head = ::(head = 1, next = Nil()), next = ::(head = ::(head = 2, next = ::(head = 3, next = Nil())), next = Nil()))
+::(head = Nil(), next = ::(head = ::(head = 1, next = Nil()), next = ::(head = ::(head = 2, next = ::(head = 3, next = Nil())), next = Nil())))