Fix #4323: allow narrowing of class type parameters in pattern match

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

@@ -984,7 +984,7 @@ class Typer extends Namer
       val accu = new TypeAccumulator[Set[Symbol]] {
         def apply(tsyms: Set[Symbol], t: Type): Set[Symbol] = {
           val tsyms1 = t match {
-            case tr: TypeRef if (tr.symbol is TypeParam) && tr.symbol.owner.isTerm && variance == 0 =>
+            case tr: TypeRef if (tr.symbol is TypeParam) && variance == 0 =>
               tsyms + tr.symbol
             case _ =>
               tsyms

tests/pos/i4323.scala

@@ -0,0 +1,9 @@
+enum Expr[T] {
+  case IExpr(value: Int) extends Expr[Int]
+  case BExpr(value: Boolean) extends Expr[Boolean]
+
+  def join(other: Expr[T]): Expr[T] = (this, other) match {
+    case (IExpr(i1), IExpr(i2)) => IExpr(i1 + i2)
+    case (BExpr(b1), BExpr(b2)) => BExpr(b1 & b2)
+  }
+}

tests/pos/i4323b.scala

@@ -0,0 +1,13 @@
+sealed trait Expr[T] {
+  import Expr._
+
+   def join(other: Expr[T]): Expr[T] = (this, other) match {
+     case (IExpr(i1), IExpr(i2)) => IExpr(i1 + i2)
+     case (BExpr(b1), BExpr(b2)) => BExpr(b1 & b2)
+   }
+}
+
+object Expr {
+  case class IExpr(value: Int) extends Expr[Int]
+  case class BExpr(value: Boolean) extends Expr[Boolean]
+}

tests/pos/i4323c.scala

@@ -0,0 +1,11 @@
+sealed trait Expr[T] { outer =>
+  class Inner {
+    def join(other: Expr[T]): Expr[T] = (outer, other) match {
+      case (IExpr(i1), IExpr(i2)) => IExpr(i1 + i2)
+      case (BExpr(b1), BExpr(b2)) => BExpr(b1 & b2)
+    }
+  }
+}
+
+case class IExpr(value: Int) extends Expr[Int]
+case class BExpr(value: Boolean) extends Expr[Boolean]