Test that no into on enums and current mirror behavior for into (#23473)

Author: odersky

tests/neg/i23400.scala

@@ -0,0 +1,27 @@
+import scala.language.experimental.into
+import Conversion.into
+
+import scala.deriving.Mirror
+
+object Opaques:
+  opaque into type MyInto[+A] >: A = A
+
+import Opaques.MyInto
+
+case class Foo(x: Int)
+case class Bar(foo: into[Foo])
+case class Baz(foo: MyInto[Foo])
+
+given Conversion[Int, Foo] = Foo(_)
+
+into enum Color: // error
+  case Red, Green
+
+def test =
+  val barMirror = summon[Mirror.Of[Bar]]
+  summon[barMirror.MirroredElemTypes =:= (into[Foo] *: EmptyTuple.type)] // error
+  summon[barMirror.MirroredElemTypes =:= (Foo *: EmptyTuple.type)] // ok
+
+  val bazMirror = summon[Mirror.Of[Baz]]
+  summon[bazMirror.MirroredElemTypes =:= (MyInto[Foo] *: EmptyTuple.type)] // ok
+  summon[bazMirror.MirroredElemTypes =:= (Foo *: EmptyTuple.type)] // error