Merge pull request scala#3496 from adriaanm/t6169

SI-6169 TODO: consolidate with fix for SI-1786 (scala#2518)

Author: gkossakowski

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -4902,6 +4902,9 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
                 )
                 arg match {
                   case Bind(_, _)                     => enhanceBounds()
+                  // TODO: consolidate fixes for SI-6169 and SI-1786 by dropping the Ident case,
+                  // in favor of doing sharpenQuantifierBounds for all ExistentialTypes, not just java-defined ones
+                  // (need to figure out how to sharpen the bounds on creation without running into cycles)
                   case Ident(name) if canEnhanceIdent => enhanceBounds()
                   case _                              =>
                 }

src/reflect/scala/reflect/internal/Types.scala

@@ -2693,6 +2693,8 @@ trait Types
      *
      *   (Also tried doing this once during class file parsing or when creating the existential type,
      *   but that causes cyclic errors because it happens too early.)
+     *
+     * TODO: figure out how to do this earlier without running into cycles, so this can subsume the fix for SI-1786
      */
     private def sharpenQuantifierBounds(): Unit = {
       /* Check that we're looking at rawToExistential's handiwork