Hide kind-polymorphism behind -Ykind-polymorphism

Author: Blaisorblade

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

@@ -123,6 +123,9 @@ class ScalaSettings extends Settings.SettingGroup {
   val YprofileRunGcBetweenPhases = PhasesSetting("-Yprofile-run-gc", "Run a GC between phases - this allows heap size to be accurate at the expense of more time. Specify a list of phases, or *", "_")
       //.withPostSetHook( _ => YprofileEnabled.value = true )
 
+  // Extremely experimetnal language feature
+  val YkindPolymorphism = BooleanSetting("-Ykind-polymorphism", "Potentially unsound.")
+
   /** Area-specific debug output */
   val YexplainLowlevel = BooleanSetting("-Yexplain-lowlevel", "When explaining type errors, show types at a lower level.")
   val YnoDoubleBindings = BooleanSetting("-Yno-double-bindings", "Assert no namedtype is bound twice (should be enabled only if program is error-free).")

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -306,7 +306,10 @@ class Definitions {
 
   lazy val AnyKindClass = {
     val cls = ctx.newCompleteClassSymbol(ScalaPackageClass, tpnme.AnyKind, AbstractFinal | Permanent, Nil)
-    if (true) cls.entered // TODO: replace with if (ctx.settings.Yanykind.value) ...
+    if (ctx.settings.YkindPolymorphism.value) {
+      // Enable kind-polymorphism by exposing scala.AnyKind
+      cls.entered
+    }
     cls
   }
   def AnyKindType = AnyKindClass.typeRef

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -102,6 +102,7 @@ class CompilationTests extends ParallelTesting {
     compileFilesInDir("tests/pos", defaultOptions) +
     compileFilesInDir("tests/pos-no-optimise", defaultOptions) +
     compileFilesInDir("tests/pos-deep-subtype", allowDeepSubtypes) +
+    compileFilesInDir("tests/pos-kind-polymorphism", defaultOptions and "-Ykind-polymorphism") +
     compileDir("tests/pos/i1137-1", defaultOptions and "-Yemit-tasty") +
     compileFile(
       // succeeds despite -Xfatal-warnings because of -nowarn
@@ -175,6 +176,7 @@ class CompilationTests extends ParallelTesting {
     compileFilesInDir("tests/neg", defaultOptions) +
     compileFilesInDir("tests/neg-tailcall", defaultOptions) +
     compileFilesInDir("tests/neg-no-optimise", defaultOptions) +
+    compileFilesInDir("tests/neg-kind-polymorphism", defaultOptions and "-Ykind-polymorphism") +
     compileFilesInDir("tests/neg-custom-args/fatal-warnings", defaultOptions.and("-Xfatal-warnings")) +
     compileFilesInDir("tests/neg-custom-args/allow-double-bindings", allowDoubleBindings) +
     compileFile("tests/neg-custom-args/i3246.scala", scala2Mode) +