Enable kind polimorphism

Enable kind polimorphism by default by replacing `-Ykind-polymorphism` with `-Yno-kind-polymorphism`.

Author: nicolasstucki

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

@@ -138,7 +138,7 @@ class ScalaSettings extends Settings.SettingGroup {
       //.withPostSetHook( _ => YprofileEnabled.value = true )
 
   // Extremely experimental language features
-  val YkindPolymorphism: Setting[Boolean] = BooleanSetting("-Ykind-polymorphism", "Enable kind polymorphism (see http://dotty.epfl.ch/docs/reference/kind-polymorphism.html). Potentially unsound.")
+  val YnoKindPolymorphism: Setting[Boolean] = BooleanSetting("-Yno-kind-polymorphism", "Enable kind polymorphism (see http://dotty.epfl.ch/docs/reference/kind-polymorphism.html). Potentially unsound.")
 
   /** Area-specific debug output */
   val YexplainLowlevel: Setting[Boolean] = BooleanSetting("-Yexplain-lowlevel", "When explaining type errors, show types at a lower level.")

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

@@ -320,7 +320,7 @@ class Definitions {
 
   lazy val AnyKindClass: ClassSymbol = {
     val cls = ctx.newCompleteClassSymbol(ScalaPackageClass, tpnme.AnyKind, AbstractFinal | Permanent, Nil)
-    if (ctx.settings.YkindPolymorphism.value) {
+    if (!ctx.settings.YnoKindPolymorphism.value) {
       // Enable kind-polymorphism by exposing scala.AnyKind
       cls.entered
     }

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

@@ -91,7 +91,6 @@ class CompilationTests extends ParallelTesting {
     compileFilesInDir("tests/pos-scala2", scala2Mode) +
     compileFilesInDir("tests/pos", defaultOptions) +
     compileFilesInDir("tests/pos-deep-subtype", allowDeepSubtypes) +
-    compileFilesInDir("tests/pos-kind-polymorphism", defaultOptions and "-Ykind-polymorphism") +
     compileFile(
       // succeeds despite -Xfatal-warnings because of -nowarn
       "tests/neg-custom-args/fatal-warnings/xfatalWarnings.scala",
@@ -145,7 +144,7 @@ class CompilationTests extends ParallelTesting {
     implicit val testGroup: TestGroup = TestGroup("compileNeg")
     compileFilesInDir("tests/neg", defaultOptions) +
     compileFilesInDir("tests/neg-tailcall", defaultOptions) +
-    compileFilesInDir("tests/neg-kind-polymorphism", defaultOptions and "-Ykind-polymorphism") +
+    compileFilesInDir("tests/neg-no-kind-polymorphism", defaultOptions and "-Yno-kind-polymorphism") +
     compileFilesInDir("tests/neg-custom-args/deprecation", defaultOptions.and("-Xfatal-warnings", "-deprecation")) +
     compileFilesInDir("tests/neg-custom-args/fatal-warnings", defaultOptions.and("-Xfatal-warnings")) +
     compileFilesInDir("tests/neg-custom-args/allow-double-bindings", allowDoubleBindings) +

docs/docs/reference/other-new-features/kind-polymorphism.md

@@ -42,5 +42,5 @@ It is declared `abstract` and `final`, so it can be neither instantiated nor ext
 
 `AnyKind` plays a special role in Scala's subtype system: It is a supertype of all other types no matter what their kind is. It is also assumed to be kind-compatible with all other types. Furthermore, `AnyKind` is treated as a higher-kinded type (so it cannot be used as a type of values), but at the same time it has no type parameters (so it cannot be instantiated).
 
-**Note**: This feature is considered experimental and is only enabled under a compiler flag
-(i.e. `-Ykind-polymorphism`). 
+**Note**: This feature is considered experimental but stable and it can be disabled under compiler flag
+(i.e. `-Yno-kind-polymorphism`).

tests/neg-no-kind-polymorphism/anykind.scala

@@ -0,0 +1,2 @@
+
+trait Foo[T <: AnyKind] // error: Not found: type AnyKind