Make opaque types decomposable

Author: nox213

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

@@ -724,7 +724,6 @@ object SpaceEngine {
         && cls.isOneOf(AbstractOrTrait) // ignore sealed non-abstract classes
         && !cls.hasAnonymousChild       // can't name anonymous classes as counter-examples
         && cls.children.nonEmpty        // can't decompose without children
-        && !sym.isOpaqueAlias           // can't instantiate subclasses to conform to an opaque type (i19275)
 
   val ListOfNoType    = List(NoType)
   val ListOfTypNoType = ListOfNoType.map(Typ(_, decomposed = true))

tests/pos/i22513.scala

@@ -0,0 +1,27 @@
+opaque type R[T] <: T = T
+
+object Test {
+  enum E:
+    case A(a: Int)
+
+  val v: R[E] = ???
+  v match
+    case E.A(_) =>
+}
+
+sealed trait Foo
+
+case class FooA() extends Foo
+case class FooB() extends Foo
+
+object O {
+  opaque type OpaqueFoo <: Foo = Foo
+  def fooB(): OpaqueFoo = FooB()
+}
+
+@main def main =
+  val p: O.OpaqueFoo = O.fooB()
+
+  p match
+    case _: FooA => println("fooA")
+    case _: FooB => println("fooB")