Fix #8649: Add regression test

compiler/test/dotc/pos-test-pickling.blacklist

@@ -37,6 +37,7 @@ i11247.scala
 i11250
 i9999.scala
 i12127.scala
+8649.scala
 
 # Opaque type
 i5720.scala

tests/pos/8649.scala

@@ -0,0 +1,32 @@
+// This is crazy:
+type Get0 = OK[Int, Unit]
+def get0: Handler[Get0] = IO[Unit]()
+
+case class HandlerAlt[A](value: Handler[A])
+
+type Handler[API] = handler.Go[API]
+
+case class IO[A]()
+case class OK[A, B]()
+
+object handler:
+  // Starter for Handler reduction:
+  type Go[API] = API match
+    case _ =>
+      HandlerSingle[API]
+
+  type HandlerSingle[X] = X match
+    case OK[_, response] =>
+      IO[response]
+
+object Minimized {
+  case class HandlerAlt[A](value: M2[A])
+
+  type M1[X] = X match {
+    case _ => M2[X]
+  }
+
+  type M2[X] = X match {
+    case Int => String
+  }
+}