Fix #12602: All sealed classes can be deecomposed

Author: liufengyun

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

@@ -659,10 +659,7 @@ class SpaceEngine(using Context) extends SpaceLogic {
       case and: AndType => canDecompose(and.tp1) || canDecompose(and.tp2)
       case _ =>
         val cls = tp.classSymbol
-        cls.is(Sealed)
-        && cls.isOneOf(AbstractOrTrait)
-        && !cls.hasAnonymousChild
-        && cls.children.nonEmpty
+        cls.is(Sealed) && !cls.hasAnonymousChild
         || cls.isAllOf(JavaEnumTrait)
         || tp.isRef(defn.BooleanClass)
         || tp.isRef(defn.UnitClass)
@@ -846,6 +843,9 @@ 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 abstract class Foo[T]
+object Foo extends Foo[Nothing]