Infer Local for val constructor params

Author: mbovel

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

@@ -440,6 +440,9 @@ object Flags {
    */
   val (PhantomSymbol @ _, _, _) = newFlags(62, "<phantom symbol>") // (could be merged with Lifted)
 
+
+  val (ValMember @ _, _, _) = newFlags(63, "<val>") // (could be merged with Lifted)
+
 // --------- Combined Flag Sets and Conjunctions ----------------------
 
   /** All possible flags */

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

@@ -2694,7 +2694,7 @@ object SymDenotations {
    *     determined by whether they have a `val` or `var` or not.
    */
   def canBeLocal(name: Name, flags: FlagSet)(using Context) =
-    !name.isConstructorName && !flags.is(Param) && !flags.is(ParamAccessor)
+    !name.isConstructorName && !flags.is(ValMember)
 
   /** Factory method for SymDenotion creation. All creations
    *  should be done via this method.

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3679,7 +3679,7 @@ object Parsers {
               if (!(mods.flags &~ (ParamAccessor | Inline | Erased | impliedMods.flags)).isEmpty)
                 syntaxError(em"`val` or `var` expected")
               if firstClause && paramOwner == ParamOwner.CaseClass then mods
-              else mods | PrivateLocal
+              else mods | PrivateLocal | ValMember
         else {
           if (isIdent(nme.inline) && in.isSoftModifierInParamModifierPosition)
             mods = addModifier(mods)

tests/neg/i22620.scala

@@ -0,0 +1,3 @@
+import scala.collection.mutable.ArrayBuffer
+
+class PrivateTest[-M](private val v: ArrayBuffer[M])

tests/pos/i22620.scala

@@ -0,0 +1,7 @@
+sealed abstract class Tree[+A](
+  final val key: A
+)
+final class RedTree[+A](key: A) extends Tree[A](key)
+object RedTree {
+  def unapply[A](t: RedTree[A]) = Some((t.key))
+}

tests/pos/i22620b.scala

@@ -36,7 +36,7 @@ package foo.test.constructors:
   case class A private (x: Int) // OK
   class B private (val x: Int) // OK
   object B { def default = B(42) }
-  class C private (private val xy: Int) // warn
+  class C private (private val xy: Int)
   object C { def default = C(42) }
   class D private (private val x: Int): // OK
     def y = x