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space: Fix how sealed abstract classes decompose #15553

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Jul 8, 2022
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space: Fix how sealed abstract classes decompose #15553

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2 changes: 1 addition & 1 deletion compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -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...
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sealed trait Baz, val x = new Baz {}, Baz.children returns Baz

Which exactly Baz in Baz.children are we talking about here?

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Sounds like a joke question 😄 . Describing a full, legal, source file:

sealed trait Baz
object Baz:
  val inst = new Baz {}

Then there are 3 classes: Baz, the interface, Baz$, the module class, and Baz$$anon$1, the anonymous subclass of Baz:

$ javap Baz.class
Compiled from "Test.scala"
public interface Baz {
  public static Baz inst();
}
$ javap Baz\$.class
Compiled from "Test.scala"
public final class Baz$ implements java.io.Serializable {
  public static final Baz$ MODULE$;
  public static {};
  public Baz inst();
}
$ javap Baz\$\$anon\$1.class
Compiled from "Test.scala"
public final class Baz$$anon$1 implements Baz {
  public Baz$$anon$1();
}

So when you look up the sealed children of Baz, then Baz itself is returned, as a representative of the anonymous subclass.

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)
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13 changes: 13 additions & 0 deletions tests/pos/i15522.scala
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Original file line number Diff line number Diff line change
@@ -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)