Eta expand $apply projected types if needed

It turns out that asSeenFrom can produce types
that get projected with $apply but that are not
higher-kinded. An exampple failure is in Iter3,
andother in scala.collection.immutable.Map (which is
now part of the test suite).

We now detect that situation, and eta expand the
projected type in `derivedSelect`, this will
force a subssequent `lookupRefined` which will give
the desired normalized type.

Also added is a configurable test that checks that
$apply projected tyeps are in fact higher-kinded.

1 files changed, 29 insertions, 0 deletions

diff --git a/src/dotty/tools/dotc/core/TypeApplications.scala b/src/dotty/tools/dotc/core/TypeApplications.scala
index d6cb3dc15..d7d205be6 100644
--- a/src/dotty/tools/dotc/core/TypeApplications.scala
+++ b/src/dotty/tools/dotc/core/TypeApplications.scala
@@ -155,6 +155,27 @@ class TypeApplications(val self: Type) extends AnyVal {
     case _ => false
   }
 
+  /** True if it can be determined without forcing that the class symbol
+   *  of this application exists and is not a lambda trait.
+   *  Equivalent to
+   *
+   *    self.classSymbol.exists && !self.classSymbol.isLambdaTrait
+   *
+   *  but without forcing anything.
+   */
+  def noHK(implicit ctx: Context): Boolean = self.stripTypeVar match {
+    case self: RefinedType =>
+      self.parent.noHK
+    case self: TypeRef =>
+      (self.denot.exists) && {
+        val sym = self.symbol
+        if (sym.isClass) !sym.isLambdaTrait
+        else sym.isCompleted && self.info.isAlias && self.info.bounds.hi.noHK
+      }
+    case _ =>
+      false
+  }
+
   /** Encode the type resulting from applying this type to given arguments */
   final def appliedTo(args: List[Type])(implicit ctx: Context): Type = /*>|>*/ track("appliedTo") /*<|<*/ {
     def matchParams(tp: Type, tparams: List[TypeSymbol], args: List[Type]): Type = args match {
@@ -510,6 +531,14 @@ class TypeApplications(val self: Type) extends AnyVal {
     if (bound.isHK && !isHK && self.typeSymbol.isClass && typeParams.nonEmpty) EtaExpand
     else self
 
+  /** Eta expand the prefix in front of any refinements. */
+  def EtaExpandCore(implicit ctx: Context): Type = self.stripTypeVar match {
+    case self: RefinedType =>
+      self.derivedRefinedType(self.parent.EtaExpandCore, self.refinedName, self.refinedInfo)
+    case _ =>
+      self.EtaExpand
+  }
+
   /** If `self` is a (potentially partially instantiated) eta expansion of type T, return T,
    *  otherwise NoType. More precisely if `self` is of the form
    *