Change private local inference to allow inferring private local for private vals constructor parameters

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

@@ -934,6 +934,9 @@ object SymDenotations {
              (  !this.is(Local)
              || isAccessPrivilegedThisType(pre)
              || canBeLocal(name, flags)
+                && {
+                  !symbol.defTree.hasAttachment(typer.Typer.OriginallyPrivateLocal)
+                }
                 && {
                   resetFlag(Local)
                   true
@@ -2658,11 +2661,9 @@ object SymDenotations {
    *  This holds for all symbols except
    *   - constructors, since they can never be referred to as members of their
    *     own, fully elaborated `this`.
-   *   - parameters and parameter accessors, since their Local status is already
-   *     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
 
   /** Factory method for SymDenotion creation. All creations
    *  should be done via this method.

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -87,6 +87,10 @@ object Typer {
   /** Tree adaptation lost fidelity; this attachment preserves the original tree. */
   val AdaptedTree = new Property.StickyKey[tpd.Tree]
 
+  /** An attachment on a tree that was originally private local
+   */
+  val OriginallyPrivateLocal = new Property.Key[Unit]
+
   /** An attachment on a Select node with an `apply` field indicating that the `apply`
    *  was inserted by the Typer.
    */
@@ -3039,6 +3043,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       report.error(em"Cannot return repeated parameter type ${sym.info.finalResultType}", sym.srcPos)
     if !sym.is(Module) && !sym.isConstructor && sym.info.finalResultType.isErasedClass then
       sym.setFlag(Erased)
+    // This is probably too late. Do this in Namer?
+    if mdef.isInstanceOf[ValDef] && sym.isAllOf(PrivateLocal) then
+      mdef.putAttachment(OriginallyPrivateLocal, ())
     mdef.ensureHasSym(sym)
     mdef.setDefTree
 

tests/pos/i22620.scala

@@ -0,0 +1,4 @@
+import scala.collection.mutable.ArrayBuffer
+
+class PrivateTest[-M](private val v: ArrayBuffer[M])
+

tests/pos/private-local-override.scala

@@ -0,0 +1,11 @@
+sealed abstract class Tree[+A](
+  final val key: A
+)
+final class RedTree[+A](key: A) extends Tree[A](key)
+final class BlackTree[+A](key: A) extends Tree[A](key)
+object RedTree {
+  def unapply[A](t: RedTree[A]) = Some((t.key))
+}
+object BlackTree {
+  def unapply[A](t: BlackTree[A]) = Some((t.key))
+}