From 37f48555ca670108b81b66c96aca5fd24e56b6b7 Mon Sep 17 00:00:00 2001
From: Liu Fengyun <liu@fengy.me>
Date: Tue, 25 May 2021 19:59:04 +0200
Subject: [PATCH] Fix #12602: All sealed classes can be deecomposed

---
 .../src/dotty/tools/dotc/transform/patmat/Space.scala     | 8 ++++----
 tests/patmat/i12602.scala                                 | 2 ++
 2 files changed, 6 insertions(+), 4 deletions(-)
 create mode 100644 tests/patmat/i12602.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
index 706071388111..65e72d2d5274 100644
--- a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
+++ b/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)}")
diff --git a/tests/patmat/i12602.scala b/tests/patmat/i12602.scala
new file mode 100644
index 000000000000..8ea53147455d
--- /dev/null
+++ b/tests/patmat/i12602.scala
@@ -0,0 +1,2 @@
+sealed abstract class Foo[T]
+object Foo extends Foo[Nothing]