Fix sneaky binary incompatibility in Discover.scala (com-lihaoyi#2752)

fixes com-lihaoyi#2749

The basic problem is that the signature of `Discover.apply()` changed,
but it only changed in the argument's generic type parameters, so it was
nominally the same type after erasure, and accepted the `Map[Class[_],
Seq]` even though it was expecting a `Map[Class[_], (Seq, Seq)]`, but
later on in the code it would blow up with a `ClassCastException`

This causes problems with Mill plugins containing external modules,
which have their own `Discover[T]` macro pre-expanded and would not get
re-compiled with the new 0.11.3 version of Mill.

This PR works around the problem by adding a forwarder
`Discover.apply(value: Map[Class, Seq])` that does the right thing and
expands the `Seq` into a `(Seq, Seq)`, while adding a dummy paramter to
`case class Discover` to avoid conflicts. A similar forwarder
`Discover.<init>(value: Map[Class, Seq])` is needed to preserve binary
compatibility (I think?)

Tested manually via `./mill -i dev.run example/basic/1-simple-scala
--import ivy:io.chris-kipp::mill-github-dependency-graph::0.2.6
io.kipp.mill.github.dependency.graph.Graph/generate`, which fails before
this PR and passes after

---------

Co-authored-by: Chris Kipp <ckipp@pm.me>
Co-authored-by: Tobias Roeser <le.petit.fou@web.de>

main/define/src/mill/define/Discover.scala

@@ -16,15 +16,38 @@ import scala.reflect.macros.blackbox
  * the `T.command` methods we find. This mapping from `Class[_]` to `MainData`
  * can then be used later to look up the `MainData` for any module.
  */
-case class Discover[T] private (val value: Map[
-  Class[_],
-  (Seq[String], Seq[mainargs.MainData[_, _]])
-])
+case class Discover[T] private (
+    value: Map[
+      Class[_],
+      (Seq[String], Seq[mainargs.MainData[_, _]])
+    ],
+    dummy: Int = 0 /* avoid conflict with Discover.apply(value: Map) below*/
+) {
+  @deprecated("Binary compatibility shim", "Mill 0.11.4")
+  private[define] def this(value: Map[Class[_], Seq[mainargs.MainData[_, _]]]) =
+    this(value.view.mapValues((Nil, _)).toMap)
+  // Explicit copy, as we also need to provide an override for bin-compat reasons
+  def copy[T](
+      value: Map[
+        Class[_],
+        (Seq[String], Seq[mainargs.MainData[_, _]])
+      ] = value,
+      dummy: Int = dummy /* avoid conflict with Discover.apply(value: Map) below*/
+  ): Discover[T] = new Discover[T](value, dummy)
+  @deprecated("Binary compatibility shim", "Mill 0.11.4")
+  private[define] def copy[T](value: Map[Class[_], Seq[mainargs.MainData[_, _]]]): Discover[T] = {
+    new Discover[T](value.view.mapValues((Nil, _)).toMap, dummy)
+  }
+}
 
 object Discover {
   def apply2[T](value: Map[Class[_], (Seq[String], Seq[mainargs.MainData[_, _]])]): Discover[T] =
     new Discover[T](value)
 
+  @deprecated("Binary compatibility shim", "Mill 0.11.4")
+  def apply[T](value: Map[Class[_], Seq[mainargs.MainData[_, _]]]): Discover[T] =
+    new Discover[T](value.view.mapValues((Nil, _)).toMap)
+
   def apply[T]: Discover[T] = macro Router.applyImpl[T]
 
   private class Router(val ctx: blackbox.Context) extends mainargs.Macros(ctx) {

main/resolve/src/mill/resolve/Resolve.scala

@@ -227,11 +227,11 @@ trait Resolve[T] {
       nullCommandDefaults: Boolean
   ): Either[String, Seq[T]] = {
     val rootResolved = ResolveCore.Resolved.Module(Segments(), rootModule.getClass)
-    val allPossibleNames = rootModule.millDiscover.value.values.flatMap(_._1).toSet
     val resolved =
       ResolveCore.resolve(rootModule, sel.value.toList, rootResolved, Segments()) match {
         case ResolveCore.Success(value) => Right(value)
         case ResolveCore.NotFound(segments, found, next, possibleNexts) =>
+          val allPossibleNames = rootModule.millDiscover.value.values.flatMap(_._1).toSet
           Left(ResolveNotFoundHandler(sel, segments, found, next, possibleNexts, allPossibleNames))
         case ResolveCore.Error(value) => Left(value)
       }