Make opaque types decomposable (scala#24068)

fixes: scala#22513

Currently, if the upper bounds of opaque types are sealed, they pass
exhaustivityCheckable, but exhaustivity checks are not handled correctly
because the compiler preventing from decomposing them.
The guard was introduced in
[scala#19368](scala#19368), but the tests
still pass even without this guard.
[Cherry-picked f4e6239]

Author: nox213

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

@@ -695,14 +695,12 @@ object SpaceEngine {
 
   extension (tp: Type)
     def isDecomposableToChildren(using Context): Boolean =
-      val sym = tp.typeSymbol  // e.g. Foo[List[Int]] = type Foo (i19275)
       val cls = tp.classSymbol // e.g. Foo[List[Int]] = class List
       tp.hasSimpleKind                  // can't decompose higher-kinded types
         && cls.is(Sealed)
         && 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")