Allow for aut-updating of package members

Previously, an overwritten package member always got the denotation of the
new package, so package members always auto-updated, and we could not even
detect that they were no longer valid. This is a problem because it means that
TermRefs and TypeRefs pointing to overwritten members will not be updated.

We now do something less sweeping. A call to `current` will invoke `bringForward`
which will do the update if the `autoUpdatePackageMembers` config flag is set.
After the call to `current` the old symbol will be considered valid, which is a
potential problems because we then have two valid symbols pointing to the same
denotations. So it would be best if we could set `autoUpdatePackageMembers = false`.

Author: odersky

compiler/src/dotty/tools/dotc/config/Config.scala

@@ -144,6 +144,11 @@ object Config {
    */
   final val simplifyApplications = true
 
+  /** If this flag is on, automatically being forward no-longer valid package members.
+   *  If it is off, raise a StaleSymbol error instead.
+   */
+  final val autoUpdatePackageMembers = true
+
   /** If set, prints a trace of all symbol completions */
   final val showCompletions = false
 

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

@@ -779,7 +779,7 @@ object Denotations {
       this match {
         case symd: SymDenotation =>
           if (ctx.stillValid(symd)) return updateValidity()
-          if (symd.owner.is(Package)) {
+          if (Config.autoUpdatePackageMembers && symd.owner.is(Package)) {
             val newd = symd.owner.info.decls.lookup(symd.name)
             if (newd.exists) return newd
           }