Infer Local for val constructor params

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

@@ -404,8 +404,8 @@ object Flags {
    *  an existentially bound symbol (Scala 2.x only) */
   val (Scala2SpecialFlags @ _, Scala2SuperAccessor @ _, Scala2Existential @ _) = newFlags(55, "<existential>")
 
-  /** Children were queried on this class */
-  val (_, _, ChildrenQueried @ _) = newFlags(56, "<children-queried>")
+  /** A class parameter that is not a `val` / Children were queried on this class */
+  val (_, NotAField @ _, ChildrenQueried @ _) = newFlags(56, "<not-a-field>", "<children-queried>")
 
   /** A module variable (Scala 2.x only) / a capture-checked class
    *  (Scala2ModuleVar is re-used as a flag for private parameter accessors in Recheck)
@@ -475,7 +475,7 @@ object Flags {
     Scala2SpecialFlags, MutableOrOpen, Opaque, Touched, JavaStatic,
     OuterOrCovariant, LabelOrContravariant, CaseAccessor, Tracked,
     Extension, NonMember, Implicit, Given, Permanent, Synthetic, Exported,
-    SuperParamAliasOrScala2x, Inline, Macro, PhantomSymbol, Invisible)
+    SuperParamAliasOrScala2x, Inline, Macro, PhantomSymbol, Invisible, NotAField)
 
   /** Flags that are not (re)set when completing the denotation, or, if symbol is
    *  a top-level class or object, when completing the denotation once the class

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(NotAField)
 
   /** 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 | NotAField
         else {
           if (isIdent(nme.inline) && in.isSoftModifierInParamModifierPosition)
             mods = addModifier(mods)

tests/pos/i22620.scala

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

tests/pos/i22620b.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/semanticdb/metac.expect

@@ -626,7 +626,7 @@ classes/C6#`<init>`().(x) => param x: Int
 classes/C6#copy$default$1(). => method copy$default$1 => Int @uncheckedVariance
 classes/C6#copy(). => method copy (param x: Int): C6
 classes/C6#copy().(x) => param x: Int
-classes/C6#x. => private val method x Int
+classes/C6#x. => private[this] val method x Int
 classes/C6. => final object C6 extends Object { self: C6.type => +4 decls }
 classes/C6.apply(). => method apply (param x: Int): C6
 classes/C6.apply().(x) => param x: Int
@@ -1957,7 +1957,7 @@ example/ImplicitConversion.newAny2stringadd#`+`(). => method + (param other: Str
 example/ImplicitConversion.newAny2stringadd#`+`().(other) => param other: String
 example/ImplicitConversion.newAny2stringadd#`<init>`(). => primary ctor <init> [typeparam A ](param self: A): newAny2stringadd[A]
 example/ImplicitConversion.newAny2stringadd#`<init>`().(self) => param self: A
-example/ImplicitConversion.newAny2stringadd#self. => private val method self A
+example/ImplicitConversion.newAny2stringadd#self. => private[this] val method self A
 example/ImplicitConversion.newAny2stringadd(). => final implicit method newAny2stringadd [typeparam A ](param self: A): newAny2stringadd[A]
 example/ImplicitConversion.newAny2stringadd().(self) => param self: A
 example/ImplicitConversion.newAny2stringadd().[A] => typeparam A 

tests/warn/i15503c.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