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.

1 files changed, 10 insertions, 2 deletions

diff --git a/src/dotty/tools/dotc/core/TypeComparer.scala b/src/dotty/tools/dotc/core/TypeComparer.scala
index bf82bfe09..14b5a403c 100644
--- a/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/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