Reinterpret ::.next$access$1 when unpickling from Scala 2

Author: nicolasstucki

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

@@ -454,6 +454,14 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
       // skip this member
       return NoSymbol
 
+    // Remove case accessor `next$access$1` and make `next` the case accessor
+    // TODO generalize to all Scala 2 accessors with this form
+    if owner == defn.ConsClass then
+      if name == nme.next then
+        flags = flags | Synthetic | CaseAccessor
+      else if name == "next$access$1".toTermName then
+        return NoSymbol
+
     name = name.adjustIfModuleClass(flags)
     if (flags.is(Method))
       name =

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),

tests/neg/i18884.scala

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

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())))