Add test for MirroredElemTypes order for Mirror.SumOf

Author: travisbrown

tests/run/deriving-constructor-order.scala

@@ -0,0 +1,24 @@
+import scala.compiletime.erasedValue
+import scala.deriving.Mirror
+
+object Test extends App {
+  inline def checkElems[A, T](using inline A: Mirror.SumOf[A]): Unit =
+    inline erasedValue[A.MirroredElemTypes] match {
+      case _: T => ()
+    }
+
+  sealed trait Base1
+  case class Foo() extends Base1
+  case object Bar extends Base1
+  case class Qux(i: Int) extends Base1
+
+  checkElems[Base1, (Foo, Bar.type, Qux)]
+
+  enum Tree[+T] {
+    case Empty
+    case Branch[T](left: Tree[T], right: Tree[T]) extends Tree[T]
+    case Leaf[T](elem: T) extends Tree[T]
+  }
+
+  checkElems[Tree[String], (Tree.Empty.type, Tree.Branch[String], Tree.Leaf[String])]
+}