From e00caba82c53d5b2c6f755f752653473bc1e9dac Mon Sep 17 00:00:00 2001
From: Yoonjae Jeon <yoonjae.jeon@devsisters.com>
Date: Thu, 25 Sep 2025 16:08:18 +0900
Subject: [PATCH 1/2] Make opaque types decomposable

---
 .../tools/dotc/transform/patmat/Space.scala   |  1 -
 tests/pos/i22513.scala                        | 27 +++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i22513.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
index 7857f8c86189..49138c7aa3aa 100644
--- a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
+++ b/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))
diff --git a/tests/pos/i22513.scala b/tests/pos/i22513.scala
new file mode 100644
index 000000000000..2cf2bc58227a
--- /dev/null
+++ b/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")

From 4224938dddc9a819796a98c2f7e8acd97849b5cd Mon Sep 17 00:00:00 2001
From: Yoonjae Jeon <yoonjae.jeon@devsisters.com>
Date: Fri, 3 Oct 2025 20:39:00 +0900
Subject: [PATCH 2/2] address review

---
 compiler/src/dotty/tools/dotc/transform/patmat/Space.scala | 1 -
 1 file changed, 1 deletion(-)

diff --git a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
index 49138c7aa3aa..685721b7d575 100644
--- a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
+++ b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
@@ -717,7 +717,6 @@ 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)