Specify order of MirroredElemTypes for Mirror.SumOf

docs/docs/reference/contextual/derivation.md

@@ -129,6 +129,10 @@ Note the following properties of `Mirror` types,
   Scala 2 versions of shapeless). Instead the collection of child types of a data type is represented by an ordinary,
   possibly parameterized, tuple type. Dotty's metaprogramming facilities can be used to work with these tuple types
   as-is, and higher level libraries can be built on top of them.
++ For both product and sum types, the elements of `MirroredElemTypes` are arranged in definition order (i.e. `Branch[T]`
+  precedes `Leaf[T]` in `MirroredElemTypes` for `Tree` because `Branch` is defined before `Leaf` in the source file).
+  This means that `Mirror.Sum` differs in this respect from shapeless's generic representation for ADTs in Scala 2,
+  where the constructors are ordered alphabetically by name.
 + The methods `ordinal` and `fromProduct` are defined in terms of `MirroredMonoType` which is the type of kind-`*`
   which is obtained from `MirroredType` by wildcarding its type parameters.
 

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])]
+}