Restrict visibility of copy and apply

Make the synthesized copy and apply methods of a case class have the
same visibility as its private constructor.

Author: odersky

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

@@ -469,7 +469,7 @@ object desugar {
           val copyRestParamss = derivedVparamss.tail.nestedMap(vparam =>
             cpy.ValDef(vparam)(rhs = EmptyTree))
           DefDef(nme.copy, derivedTparams, copyFirstParams :: copyRestParamss, TypeTree(), creatorExpr)
-            .withMods(synthetic) :: Nil
+            .withFlags(Synthetic | constr1.mods.flags & AccessFlags) :: Nil
         }
       }
 
@@ -574,7 +574,7 @@ object desugar {
           if (mods is Abstract) Nil
           else
             DefDef(nme.apply, derivedTparams, derivedVparamss, applyResultTpt, widenedCreatorExpr)
-              .withFlags(Synthetic | (constr1.mods.flags & DefaultParameterized)) :: widenDefs
+              .withFlags(Synthetic | constr1.mods.flags & (DefaultParameterized | AccessFlags)) :: widenDefs
         val unapplyMeth = {
           val unapplyParam = makeSyntheticParameter(tpt = classTypeRef)
           val unapplyRHS = if (arity == 0) Literal(Constant(true)) else Ident(unapplyParam.name)

tests/neg/private-case-class-constr.scala

@@ -0,0 +1,10 @@
+case class Nat private (value: Int)
+object Nat {
+  def apply(value: Int, disable: Boolean): Option[Nat] =
+    if (value < 0 || disable) None else Some(new Nat(value))
+}
+object Test {
+  val n1o = Nat(2) // error
+  val n2o = Nat(2, false) // ok
+  for (n <- n2o) yield n.copy() // error
+}