Fix #12602: Only generate sum mirrors for sealed trait or sealed abstract class

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/SymUtils.scala

@@ -100,6 +100,8 @@ object SymUtils:
     def whyNotGenericSum(declScope: Symbol)(using Context): String =
       if (!self.is(Sealed))
         s"it is not a sealed ${self.kindString}"
+      else if (!self.isOneOf(AbstractOrTrait))
+        s"it is not an abstract class"
       else {
         val children = self.children
         val companionMirror = self.useCompanionAsMirror

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -846,6 +846,9 @@ class SpaceEngine(using Context) extends SpaceLogic {
 
     if (!exhaustivityCheckable(sel)) return
 
+    debug.println("checking " + _match.show)
+    debug.println("selTyp = " + selTyp.show)
+
     val patternSpace = Or(cases.foldLeft(List.empty[Space]) { (acc, x) =>
       val space = if (x.guard.isEmpty) project(x.pat) else Empty
       debug.println(s"${x.pat.show} ====> ${show(space)}")

tests/patmat/i12602.scala

@@ -0,0 +1,2 @@
+sealed class Foo[T]
+object Foo extends Foo[Nothing]