Add Typeable test

tests/run/Typeable.check

@@ -0,0 +1,8 @@
+1 is a Int
+List(1, 2, 3) is a List[Int]
+List(1, 2, 3) is not a Int
+1 is not a List[Int]
+a is not a Int
+List() is a List[Int]
+List(1, a) is not a List[Int]
+List(1, 2) is a List[Int]

tests/run/Typeable.scala

@@ -0,0 +1,57 @@
+/** A test that shows how to use shapeless.Typeable in extractors without the
+ *  TypeLevel Scala 4 extensions.
+ *  In essence you have to write
+ *
+ *      case Typeable.instanceOf[T](x) =>
+ *
+ *  instead of
+ *
+ *      case Typeable[T](x)
+ *
+ *  The first idiom would be nice to have but it requires more backtracking
+ *  in Typer that we allow now. Essentially, given
+ *
+ *      case C[T](x)
+ *
+ *  it's unclear whether this should expand to `C[T].unapply(x)`, (as it does now)
+ *  or to `C.unapply[T](x)` (which is what TypeLevel Scala 4 did, I believe)
+ */
+trait Typeable[T]:
+  def cast(x: Any): Option[T]
+  def describe: String
+  override def toString = s"Typeable[$describe]"
+
+object Typeable:
+  def apply[T: Typeable]: Typeable[T] = summon
+
+  class instanceOf[T: Typeable]:
+    def unapply(x: Any): Option[T] = Typeable[T].cast(x)
+
+  given int as Typeable[Int]:
+    def cast(x: Any): Option[Int] = x match
+      case x: Int => Some(x)
+      case _ => None
+    def describe = "Int"
+
+  given list[T: Typeable] as Typeable[List[T]]:
+    def cast(x: Any): Option[List[T]] = x match
+      case x: List[_] if x.forall(Typeable[T].cast(_).isDefined) => Some(x.asInstanceOf[List[T]])
+      case _ => None
+    def describe = s"List[${Typeable[T].describe}]"
+end Typeable
+
+def testInstance[T: Typeable](x: Any): Unit =
+  val isa = x match
+    case Typeable.instanceOf[T](x) => "is a"
+    case _ => "is not a"
+  println(s"$x $isa ${Typeable[T].describe}")
+
+@main def Test() =
+  testInstance[Int](1)
+  testInstance[List[Int]](List(1, 2, 3))
+  testInstance[Int](List(1, 2, 3))
+  testInstance[List[Int]](1)
+  testInstance[Int]("a")
+  testInstance[List[Int]](Nil)
+  testInstance[List[Int]](1 :: "a" :: Nil)
+  testInstance[List[Int]](1 :: 2 :: Nil)