Perform variance adaptation only when needed in isSubType

Previously adaptIfHK was performed on every type application. This made
t3152 fail. We now do this only on demand, in isSubType. t3152 now passes
again. But the change unmasked another error, which makes Iter2 fail to compile.

Author: odersky

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

@@ -167,11 +167,11 @@ object TypeApplications {
     }
   }
 
-   /** Adapt all arguments to possible higher-kinded type parameters using adaptIfHK
+   /** Adapt all arguments to possible higher-kinded type parameters using etaExpandIfHK
    */
-  def adaptArgs(tparams: List[Symbol], args: List[Type])(implicit ctx: Context): List[Type] =
+  def etaExpandIfHK(tparams: List[Symbol], args: List[Type])(implicit ctx: Context): List[Type] =
     if (tparams.isEmpty) args
-    else args.zipWithConserve(tparams)((arg, tparam) => arg.adaptIfHK(tparam.infoOrCompleter))
+    else args.zipWithConserve(tparams)((arg, tparam) => arg.etaExpandIfHK(tparam.infoOrCompleter))
 
   def argRefs(rt: RefinedType, n: Int)(implicit ctx: Context) =
     List.range(0, n).map(i => RefinedThis(rt).select(tpnme.hkArg(i)))
@@ -340,11 +340,21 @@ class TypeApplications(val self: Type) extends AnyVal {
       self.EtaExpand(self.typeParams)
   }
 
-   /** Adapt argument A to type parameter P in the case P is higher-kinded.
-   *  This means:
-   *  (1) Make sure that A is a type lambda, if necessary by eta-expanding it.
-   *  (2) Make sure the variances of the type lambda
-   *  agrees with variances of corresponding higherkinded type parameters. Example:
+  /** Eta expand if `self` is a (non-lambda) class reference and `bound` is a higher-kinded type */
+  def etaExpandIfHK(bound: Type)(implicit ctx: Context): Type = {
+    val boundLambda = bound.LambdaTrait
+    val hkParams = boundLambda.typeParams
+    if (hkParams.isEmpty) self
+    else self match {
+      case self: TypeRef if self.symbol.isClass && self.typeParams.length == hkParams.length =>
+        EtaExpansion(self)
+      case _ => self
+    }
+  }
+
+  /** If argument A and type parameter P are higher-kinded, adapt the variances
+   *  of A to those of P, ensuring that  the variances of the type lambda A
+   *  agree with the variances of corresponding higherkinded type parameters of P. Example:
    *
    *     class Companion[+CC[X]]
    *     Companion[List]
@@ -367,25 +377,26 @@ class TypeApplications(val self: Type) extends AnyVal {
    *  and the second is not a subtype of the first. So if we have overridding memebrs of the two
    *  types we get an error.
    */
-  def adaptIfHK(bound: Type)(implicit ctx: Context): Type = {
+  def adaptHkVariances(bound: Type)(implicit ctx: Context): Type = {
     val boundLambda = bound.LambdaTrait
     val hkParams = boundLambda.typeParams
     if (hkParams.isEmpty) self
-    else self match {
-      case self: TypeRef if self.symbol.isClass && self.typeParams.length == hkParams.length =>
-        EtaExpansion(self).adaptIfHK(bound)
-      case _ =>
-        def adaptArg(arg: Type): Type = arg match {
-          case arg: TypeRef
-          if arg.symbol.isLambdaTrait &&
-             !arg.symbol.typeParams.corresponds(boundLambda.typeParams)(_.variance == _.variance) =>
-            arg.prefix.select(boundLambda)
-          case arg: RefinedType =>
-            arg.derivedRefinedType(adaptArg(arg.parent), arg.refinedName, arg.refinedInfo)
-          case _ =>
-            arg
-        }
-        adaptArg(self)
+    else {
+      def adaptArg(arg: Type): Type = arg match {
+        case arg: TypeRef if arg.symbol.isLambdaTrait &&
+          !arg.symbol.typeParams.corresponds(hkParams)(_.variance == _.variance) &&
+          arg.symbol.typeParams.corresponds(hkParams)(varianceConforms) =>
+          arg.prefix.select(boundLambda)
+        case arg: RefinedType =>
+          arg.derivedRefinedType(adaptArg(arg.parent), arg.refinedName, arg.refinedInfo)
+        case arg @ TypeAlias(alias) =>
+          arg.derivedTypeAlias(adaptArg(alias))
+        case arg @ TypeBounds(lo, hi) =>
+          arg.derivedTypeBounds(lo, adaptArg(hi))
+        case _ =>
+          arg
+      }
+      adaptArg(self)
     }
   }
 

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

@@ -703,11 +703,19 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
    *       Further, no refinement refers back to the refined type via a refined this.
    *  The precondition is established by `skipMatching`.
    */
-  private def isSubRefinements(tp1: RefinedType, tp2: RefinedType, limit: Type): Boolean =
-    isSubType(tp1.refinedInfo, tp2.refinedInfo) && (
+  private def isSubRefinements(tp1: RefinedType, tp2: RefinedType, limit: Type): Boolean = {
+    def hasSubRefinement(tp1: RefinedType, refine2: Type): Boolean = {
+      isSubType(tp1.refinedInfo, refine2) || {
+        // last effort: try to adapt variances of higher-kinded types if this is sound.
+        val adapted2 = refine2.adaptHkVariances(tp1.parent.member(tp1.refinedName).symbol.info)
+        adapted2.ne(refine2) && hasSubRefinement(tp1, adapted2)
+      }
+    }
+    hasSubRefinement(tp1, tp2.refinedInfo) && (
       (tp2.parent eq limit) ||
       isSubRefinements(
         tp1.parent.asInstanceOf[RefinedType], tp2.parent.asInstanceOf[RefinedType], limit))
+  }
 
   /** A type has been covered previously in subtype checking if it
    *  is some combination of TypeRefs that point to classes, where the

src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -715,7 +715,7 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
           else TypeRef(pre, sym.name.asTypeName)
         val args = until(end, readTypeRef)
         if (sym == defn.ByNameParamClass2x) ExprType(args.head)
-        else if (args.nonEmpty) tycon.safeAppliedTo(adaptArgs(sym.typeParams, args))
+        else if (args.nonEmpty) tycon.safeAppliedTo(etaExpandIfHK(sym.typeParams, args))
         else if (sym.typeParams.nonEmpty) tycon.EtaExpand(sym.typeParams)
         else tycon
       case TYPEBOUNDStpe =>

src/dotty/tools/dotc/typer/Applications.scala

@@ -614,7 +614,7 @@ trait Applications extends Compatibility { self: Typer =>
   }
 
   def adaptTypeArg(tree: tpd.Tree, bound: Type)(implicit ctx: Context): tpd.Tree =
-    tree.withType(tree.tpe.adaptIfHK(bound))
+    tree.withType(tree.tpe.etaExpandIfHK(bound))
 
   /** Rewrite `new Array[T](....)` trees to calls of newXYZArray methods. */
   def convertNewArray(tree: tpd.Tree)(implicit ctx: Context): tpd.Tree = tree match {

src/dotty/tools/dotc/typer/Namer.scala

@@ -853,7 +853,7 @@ class Namer { typer: Typer =>
           val tycon = tp.withoutArgs(args)
           val tycon1 = this(tycon)
           val tparams = tycon.typeParams
-          val args1 = if (args.length == tparams.length) adaptArgs(tparams, args) else args
+          val args1 = if (args.length == tparams.length) etaExpandIfHK(tparams, args) else args
           if ((tycon1 eq tycon) && (args1 eq args)) tp else tycon1.appliedTo(args1)
         } else mapOver(tp)
       case _ => mapOver(tp)