Change hk take 2

src/dotty/tools/dotc/config/Config.scala

@@ -71,6 +71,11 @@ object Config {
   /** If this flag is set, take the fast path when comparing same-named type-aliases and types */
   final val fastPathForRefinedSubtype = true
 
+  /** If this flag is set, $apply projections are checked that they apply to a
+   *  higher-kinded type.
+   */
+  final val checkProjections = false
+
   /** When set, use new signature-based matching.
    *  Advantage of doing so: It's supposed to be faster
    *  Disadvantage: It might hide inconsistencies, so while debugging it's better to turn it off

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

@@ -494,7 +494,7 @@ class Definitions {
   /** The set of HigherKindedXYZ traits encountered so far */
   def lambdaTraits: Set[Symbol] = myLambdaTraits
 
-  private var lambdaTraitForVariances = mutable.Map[List[Int], ClassSymbol]()
+  private var LambdaTraitForVariances = mutable.Map[List[Int], ClassSymbol]()
 
   /** The HigherKinded trait corresponding to symbols `boundSyms` (which are assumed
    *  to be the type parameters of a higher-kided type). This is a class symbol that
@@ -513,7 +513,7 @@ class Definitions {
    *   - for each positive or negative variance v_i there is a parent trait Pj which
    *     is the same as LambdaXYZ except that it has `I` in i-th position.
    */
-  def lambdaTrait(vcs: List[Int]): ClassSymbol = {
+  def LambdaTrait(vcs: List[Int]): ClassSymbol = {
     assert(vcs.nonEmpty)
 
     def varianceFlags(v: Int) = v match {
@@ -527,17 +527,17 @@ class Definitions {
         val cls = denot.asClass.classSymbol
         val paramDecls = newScope
         for (i <- 0 until vcs.length)
-          newTypeParam(cls, tpnme.lambdaArgName(i), varianceFlags(vcs(i)), paramDecls)
-        newTypeField(cls, tpnme.Apply, Covariant, paramDecls)
+          newTypeParam(cls, tpnme.LambdaArgName(i), varianceFlags(vcs(i)), paramDecls)
+        newTypeField(cls, tpnme.hkApply, Covariant, paramDecls)
         val parentTraitRefs =
           for (i <- 0 until vcs.length if vcs(i) != 0)
-          yield lambdaTrait(vcs.updated(i, 0)).typeRef
+          yield LambdaTrait(vcs.updated(i, 0)).typeRef
         denot.info = ClassInfo(
             ScalaPackageClass.thisType, cls, ObjectClass.typeRef :: parentTraitRefs.toList, paramDecls)
       }
     }
 
-    val traitName = tpnme.lambdaTraitName(vcs)
+    val traitName = tpnme.LambdaTraitName(vcs)
 
     def createTrait = {
       val cls = newClassSymbol(
@@ -549,7 +549,7 @@ class Definitions {
       cls
     }
 
-    lambdaTraitForVariances.getOrElseUpdate(vcs, createTrait)
+    LambdaTraitForVariances.getOrElseUpdate(vcs, createTrait)
   }
 
   // ----- primitive value class machinery ------------------------------------------

src/dotty/tools/dotc/core/NameOps.scala

@@ -110,7 +110,7 @@ object NameOps {
     /** The index of the higher-kinded type parameter with this name.
      *  Pre: isLambdaArgName.
      */
-    def lambdaArgIndex: Int =
+    def LambdaArgIndex: Int =
       name.drop(tpnme.LAMBDA_ARG_PREFIX.length).toString.toInt
 
     /** If the name ends with $nn where nn are

src/dotty/tools/dotc/core/StdNames.scala

@@ -173,7 +173,7 @@ object StdNames {
     final val WILDCARD_STAR: N                  = "_*"
     final val REIFY_TREECREATOR_PREFIX: N       = "$treecreator"
     final val REIFY_TYPECREATOR_PREFIX: N       = "$typecreator"
-    final val LAMBDA_ARG_PREFIX: N              = "HK$"
+    final val LAMBDA_ARG_PREFIX: N              = "hk$"
     final val LAMBDA_ARG_PREFIXhead: Char       = LAMBDA_ARG_PREFIX.head
 
     final val Any: N             = "Any"
@@ -311,7 +311,7 @@ object StdNames {
 
     val AnnotatedType: N        = "AnnotatedType"
     val AppliedTypeTree: N      = "AppliedTypeTree"
-    val Apply: N                = "Apply"
+    val hkApply: N              = "$apply"
     val ArrayAnnotArg: N        = "ArrayAnnotArg"
     val Constant: N             = "Constant"
     val ConstantType: N         = "ConstantType"
@@ -739,8 +739,8 @@ object StdNames {
     def syntheticTypeParamNames(num: Int): List[TypeName] =
       (0 until num).map(syntheticTypeParamName)(breakOut)
 
-    def lambdaTraitName(vcs: List[Int]): TypeName = LambdaPrefix ++ vcs.map(varianceSuffix).mkString
-    def lambdaArgName(n: Int) = LAMBDA_ARG_PREFIX ++ n.toString
+    def LambdaTraitName(vcs: List[Int]): TypeName = LambdaPrefix ++ vcs.map(varianceSuffix).mkString
+    def LambdaArgName(n: Int) = LAMBDA_ARG_PREFIX ++ n.toString
 
     final val Conforms = encode("<:<")
 

src/dotty/tools/dotc/core/Substituters.scala

@@ -277,4 +277,31 @@ trait Substituters { this: Context =>
   final class SubstParamsMap(from: BindingType, to: List[Type]) extends DeepTypeMap {
     def apply(tp: Type) = substParams(tp, from, to, this)
   }
+
+  /** A map for "cycle safe substitutions" which do not force the denotation
+   *  of a TypeRef unless the name matches up with one of the substituted symbols.
+   */
+  final class SafeSubstMap(from: List[Symbol], to: List[Type]) extends TypeMap {
+    def apply(tp: Type): Type = tp match {
+      case tp: NamedType =>
+        try {
+          var sym: Symbol = null
+          var fs = from
+          var ts = to
+          while (fs.nonEmpty) {
+            if (fs.head.name == tp.name) {
+              if (sym == null) sym = tp.symbol
+              if (fs.head eq sym) return ts.head
+            }
+            fs = fs.tail
+            ts = ts.tail
+          }
+          tp.newLikeThis(apply(tp.prefix))
+        }
+        catch {
+          case ex: CyclicReference => tp.derivedSelect(apply(tp.prefix))
+        }
+      case _ => mapOver(tp)
+    }
+  }
 }