Handle -Ykind-projector as legacy option

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -400,7 +400,7 @@ private sealed trait YSettings:
   val YstopBefore: Setting[List[String]] = PhasesSetting(ForkSetting, "Ystop-before", "Stop before") // stop before erasure as long as we have not debugged it fully
   val YshowSuppressedErrors: Setting[Boolean] = BooleanSetting(ForkSetting, "Yshow-suppressed-errors", "Also show follow-on errors and warnings that are normally suppressed.")
   val YdetailedStats: Setting[Boolean] = BooleanSetting(ForkSetting, "Ydetailed-stats", "Show detailed internal compiler stats (needs Stats.enabled to be set to true).")
-  val YkindProjector: Setting[String] = ChoiceSetting(ForkSetting, "Ykind-projector", "[underscores, enable, disable]", "Allow `*` as type lambda placeholder to be compatible with kind projector. When invoked as -Ykind-projector:underscores will repurpose `_` to be a type parameter placeholder, this will disable usage of underscore as a wildcard.", List("disable", "enable", "underscores"), "disable")
+  val YkindProjector: Setting[String] = LegacyChoiceSetting(ForkSetting, "Ykind-projector", "[underscores, enable, disable]", "Allow `*` as type lambda placeholder to be compatible with kind projector. When invoked as -Ykind-projector:underscores will repurpose `_` to be a type parameter placeholder, this will disable usage of underscore as a wildcard.", List("disable", "", "underscores"), "disable")
   val YprintPos: Setting[Boolean] = BooleanSetting(ForkSetting, "Yprint-pos", "Show tree positions.")
   val YprintPosSyms: Setting[Boolean] = BooleanSetting(ForkSetting, "Yprint-pos-syms", "Show symbol definitions positions.")
   val YnoDeepSubtypes: Setting[Boolean] = BooleanSetting(ForkSetting, "Yno-deep-subtypes", "Throw an exception on deep subtyping call stacks.")

compiler/src/dotty/tools/dotc/config/Settings.scala

@@ -71,7 +71,9 @@ object Settings:
     depends: List[(Setting[?], Any)] = Nil,
     ignoreInvalidArgs: Boolean = false,
     propertyClass: Option[Class[?]] = None,
-    deprecationMsg: Option[String] = None)(private[Settings] val idx: Int) {
+    deprecationMsg: Option[String] = None,
+    // kept only for -Ykind-projector option compatibility
+    legacyArgs: Boolean = false)(private[Settings] val idx: Int) {
 
 
     assert(name.startsWith(s"-$category"), s"Setting $name does not start with category -$category")
@@ -188,7 +190,7 @@ object Settings:
           case (OptionTag, _) =>
             update(Some(propertyClass.get.getConstructor().newInstance()), args)
           case (ct, args) =>
-            val argInArgRest = !argRest.isEmpty
+            val argInArgRest = !argRest.isEmpty || (legacyArgs && choices.exists(_.contains("")))
             val argAfterParam = !argInArgRest && args.nonEmpty && (ct == IntTag || !args.head.startsWith("-"))
             if argInArgRest then
               doSetArg(argRest, args)
@@ -336,6 +338,10 @@ object Settings:
     def ChoiceSetting(category: String, name: String, helpArg: String, descr: String, choices: List[String], default: String, aliases: List[String] = Nil): Setting[String] =
       publish(Setting(category, validateAndPrependName(name), descr, default, helpArg, Some(choices), aliases = aliases.map(validateSetting)))
 
+    // Allows only args after :, but supports empty string as a choice. Used for -Ykind-projector
+    def LegacyChoiceSetting(category: String, name: String, helpArg: String, descr: String, choices: List[String], default: String, aliases: List[String] = Nil): Setting[String] =
+      publish(Setting(category, validateAndPrependName(name), descr, default, helpArg, Some(choices), aliases = aliases.map(validateSetting), legacyArgs = true))
+
     def MultiChoiceSetting(category: String, name: String, helpArg: String, descr: String, choices: List[String], default: List[String], aliases: List[String] = Nil): Setting[List[String]] =
       publish(Setting(category, validateAndPrependName(name), descr, default, helpArg, Some(choices), aliases = aliases.map(validateSetting)))
 

tests/neg/kind-projector.scala

@@ -1,4 +1,4 @@
-//> using options -Ykind-projector:enable
+//> using options -Ykind-projector
 
 package kind_projector_neg
 

tests/pos/kind-projector.scala

@@ -1,4 +1,4 @@
-//> using options -Ykind-projector:enable
+//> using options -Ykind-projector
 
 package kind_projector