Fix #5857: handle qualified private/protected case class constructor

- apply inherits qualified private access modifiers
- copy inherits qualified private/protected access modifiers

Author: Jasper-M

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -499,7 +499,7 @@ object desugar {
           val copyRestParamss = derivedVparamss.tail.nestedMap(vparam =>
             cpy.ValDef(vparam)(rhs = EmptyTree))
           DefDef(nme.copy, derivedTparams, copyFirstParams :: copyRestParamss, TypeTree(), creatorExpr)
-            .withFlags(Synthetic | constr1.mods.flags & copiedAccessFlags) :: Nil
+            .withMods(Modifiers(Synthetic | constr1.mods.flags & copiedAccessFlags, constr1.mods.privateWithin)) :: Nil
         }
       }
 
@@ -577,11 +577,16 @@ object desugar {
           else
             enumApplyResult(cdef, parents, derivedEnumParams, appliedRef(enumClassRef, derivedEnumParams))
 
+        def applyShouldInheritAccess = {
+          val mods = constr1.mods
+          mods.is(Private) || (!mods.is(Protected) && mods.hasPrivateWithin)
+        }
+
         val companionParent =
           if (constrTparams.nonEmpty ||
               constrVparamss.length > 1 ||
               mods.is(Abstract) ||
-              constr.mods.is(Private)) anyRef
+              applyShouldInheritAccess) anyRef
           else
             // todo: also use anyRef if constructor has a dependent method type (or rule that out)!
             (constrVparamss :\ (if (isEnumCase) applyResultTpt else classTypeRef)) (
@@ -592,8 +597,13 @@ object desugar {
           if (mods is Abstract) Nil
           else {
             val copiedFlagsMask = DefaultParameterized | (copiedAccessFlags & Private)
+            val appMods = {
+              val mods = Modifiers(Synthetic | constr1.mods.flags & copiedFlagsMask)
+              if (applyShouldInheritAccess) mods.withPrivateWithin(constr1.mods.privateWithin)
+              else mods
+            }
             val app = DefDef(nme.apply, derivedTparams, derivedVparamss, applyResultTpt, widenedCreatorExpr)
-              .withFlags(Synthetic | constr1.mods.flags & copiedFlagsMask)
+              .withMods(appMods)
             app :: widenDefs
           }
         val unapplyMeth = {

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

@@ -123,7 +123,7 @@ object Comments {
         tree match {
           case tree: untpd.DefDef =>
             val newName = ctx.freshNames.newName(tree.name, NameKinds.DocArtifactName)
-            tree.copy(name = newName)
+            untpd.cpy.DefDef(tree)(name = newName)
           case _ =>
             ctx.error(ProperDefinitionNotFound(), ctx.source.atSpan(codePos))
             tree

compiler/src/dotty/tools/dotc/printing/DecompilerPrinter.scala

@@ -70,7 +70,7 @@ class DecompilerPrinter(_ctx: Context) extends RefinedPrinter(_ctx) {
       val bodyText = " {" ~~ toTextGlobal(impl.body, "\n") ~ "}"
       parentsText.provided(parents.nonEmpty) ~ bodyText
     }
-    else super.toTextTemplate(impl.copy(parentsOrDerived = parents, preBody = body), ofNew)
+    else super.toTextTemplate(untpd.cpy.Template(impl)(parents = parents, body = body), ofNew)
   }
 
   override protected def typeApplyText[T >: Untyped](tree: TypeApply[T]): Text = {

tests/neg/i5857.scala

@@ -0,0 +1,16 @@
+object qux {
+  case class Foo private[qux] (a: Int)
+  val foo0 = Foo(0) // ok
+  val foo1 = foo0.copy(1) // ok
+
+  case class Bar protected[qux] (a: Int)
+  val bar0 = Bar(0) // ok
+  val bar1 = bar0.copy(1) // ok
+}
+
+object Test {
+  val foo2 = qux.Foo(2) // error
+  val foo3 = qux.foo0.copy(3) // error
+  val bar2 = qux.Bar(2) // ok
+  val bar3 = qux.bar0.copy(3) // error
+}