Update comments on what does not work yet

Author: odersky

compiler/src/dotty/tools/dotc/core/Flags.scala

@@ -18,6 +18,10 @@ object Flags {
   }
   type FlagSet = opaques.FlagSet
   def FlagSet(bits: Long): FlagSet = opaques.FlagSet(bits)
+  // DOTTY problem: would like to replace previous 2 lines with
+  //   export opaques.FlagSet
+  // but this makes `def FlagSet` in `opaques` ill-typed.
+
 
   delegate FlagOps {
 

compiler/src/dotty/tools/dotc/tastyreflect/KernelImpl.scala

@@ -17,7 +17,7 @@ import dotty.tools.dotc.util.SourceFile
 
 import scala.tasty.reflect.Kernel
 
-import delegate Flags.FlagOps // DOTTY problem: this should not be needed as we should include prefixes of aliases in implicit scopes
+import delegate Flags.FlagOps // DOTTY problem: this line can be dropped in 0.17 once #6712 is in bootstrap.
 
 class KernelImpl(val rootContext: core.Contexts.Context, val rootPosition: util.SourcePosition) extends Kernel {
 

compiler/src/dotty/tools/dotc/typer/Variances.scala

@@ -3,7 +3,7 @@ package typer
 
 import core._
 import Types._, Contexts._, Flags._, Symbols._, Annotations._
-import delegate Flags.FlagOps // DOTTY problem: this should not be needed as we should include prefixes of aliases in implicit scopes
+import delegate Flags.FlagOps // DOTTY problem: this line can be dropped in 0.17 once #6712 is in bootstrap.
 
 object Variances {