space: Fix how sealed abstract classes decompose

Author: dwijnand

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

@@ -631,7 +631,7 @@ class SpaceEngine(using Context) extends SpaceLogic {
       case tp =>
         def getChildren(sym: Symbol): List[Symbol] =
           sym.children.flatMap { child =>
-            if child eq sym then Nil // i3145: sealed trait Baz, val x = new Baz {}, Baz.children returns Baz...
+            if child eq sym then List(sym) // i3145: sealed trait Baz, val x = new Baz {}, Baz.children returns Baz...
             else if tp.classSymbol == defn.TupleClass || tp.classSymbol == defn.NonEmptyTupleClass then
               List(child) // TupleN and TupleXXL classes are used for Tuple, but they aren't Tuple's children
             else if (child.is(Private) || child.is(Sealed)) && child.isOneOf(AbstractOrTrait) then getChildren(child)

tests/pos/i15522.scala

@@ -0,0 +1,13 @@
+// scalac: -Werror
+sealed trait Coverage
+sealed abstract case class Range(min: Double, max: Double) extends Coverage
+case object Empty extends Coverage
+
+object Test:
+  def mkCoverage(min: Double, max: Double): Coverage =
+    if min < max then new Range(min, max) {} else Empty
+
+  def meth(c1: Coverage, c2: Coverage): Coverage = (c1, c2) match
+    case (Empty, _)                     => Empty
+    case (_, Empty)                     => Empty // was: Unreachable case
+    case (Range(a1, b1), Range(a2, b2)) => mkCoverage(a1 max a2, b1 min b2)