3 files changed, 35 insertions, 10 deletions

diff --git a/src/dotty/tools/dotc/core/Denotations.scala b/src/dotty/tools/dotc/core/Denotations.scala
index 6502c4a40..756e0b02d 100644
--- a/src/dotty/tools/dotc/core/Denotations.scala
+++ b/src/dotty/tools/dotc/core/Denotations.scala
@@ -2,7 +2,7 @@ package dotty.tools
 package dotc
 package core
 
-import SymDenotations.{ SymDenotation, ClassDenotation, NoDenotation }
+import SymDenotations.{ SymDenotation, ClassDenotation, NoDenotation, NotDefinedHereDenotation }
 import Contexts.{Context, ContextBase}
 import Names.{Name, PreName}
 import Names.TypeName
@@ -128,7 +128,17 @@ object Denotations {
      */
     def atSignature(sig: Signature, site: Type = NoPrefix)(implicit ctx: Context): SingleDenotation
 
-    /** The variant of this denotation that's current in the given context. */
+    /** The variant of this denotation that's current in the given context, or
+     *  `NotDefinedHereDenotation` if this denotation does not exist at current phase, but
+     *  is defined elsewhere in this run.
+     */
+    def currentIfExists(implicit ctx: Context): Denotation
+
+    /** The variant of this denotation that's current in the given context.
+     *  If no such denotation exists: If Mode.FutureDefs is set, the
+     *  denotation with each alternative at its first point of definition,
+     *  otherwise a `NotDefinedHere` exception is thrown.
+     */
     def current(implicit ctx: Context): Denotation
 
     /** Is this denotation different from NoDenotation or an ErrorDenotation? */
@@ -349,6 +359,8 @@ object Denotations {
     final def signature(implicit ctx: Context) = Signature.OverloadedSignature
     def atSignature(sig: Signature, site: Type)(implicit ctx: Context): SingleDenotation =
       denot1.atSignature(sig, site) orElse denot2.atSignature(sig, site)
+    def currentIfExists(implicit ctx: Context): Denotation =
+      derivedMultiDenotation(denot1.currentIfExists, denot2.currentIfExists)
     def current(implicit ctx: Context): Denotation =
       derivedMultiDenotation(denot1.current, denot2.current)
     def altsWith(p: Symbol => Boolean): List[SingleDenotation] =
@@ -530,7 +542,7 @@ object Denotations {
      *  is brought forward to be valid in the new runId. Otherwise
      *  the symbol is stale, which constitutes an internal error.
      */
-    def current(implicit ctx: Context): SingleDenotation = {
+    def currentIfExists(implicit ctx: Context): SingleDenotation = {
       val currentPeriod = ctx.period
       val valid = myValidFor
       if (valid.code <= 0) {
@@ -593,17 +605,24 @@ object Denotations {
             //println(s"searching: $cur at $currentPeriod, valid for ${cur.validFor}")
             cur = cur.nextInRun
             cnt += 1
-            if (cnt > MaxPossiblePhaseId)
-              if (ctx.mode is Mode.FutureDefsOK)
-                return current(ctx.withPhase(coveredInterval.firstPhaseId))
-              else
-                throw new NotDefinedHere(demandOutsideDefinedMsg)
+            if (cnt > MaxPossiblePhaseId) return NotDefinedHereDenotation
           }
           cur
         }
       }
     }
 
+    def current(implicit ctx: Context): SingleDenotation = {
+      val d = currentIfExists
+      if (d ne NotDefinedHereDenotation) d else currentNoDefinedHere
+    }
+
+    private def currentNoDefinedHere(implicit ctx: Context): SingleDenotation =
+      if (ctx.mode is Mode.FutureDefsOK)
+        current(ctx.withPhase(coveredInterval.firstPhaseId))
+      else
+        throw new NotDefinedHere(demandOutsideDefinedMsg)
+
     private def demandOutsideDefinedMsg(implicit ctx: Context): String =
       s"demanding denotation of $this at phase ${ctx.phase}(${ctx.phaseId}) outside defined interval: defined periods are${definedPeriodsString}"
 
diff --git a/src/dotty/tools/dotc/core/SymDenotations.scala b/src/dotty/tools/dotc/core/SymDenotations.scala
index 275fb8257..b93585324 100644
--- a/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -1650,7 +1650,7 @@ object SymDenotations {
     }
   }
 
-  object NoDenotation extends SymDenotation(
+  class NoDenotation extends SymDenotation(
     NoSymbol, NoSymbol, "<none>".toTermName, Permanent, NoType) {
     override def exists = false
     override def isTerm = false
@@ -1660,6 +1660,9 @@ object SymDenotations {
     validFor = Period.allInRun(NoRunId) // will be brought forward automatically
   }
 
+  val NoDenotation = new NoDenotation
+  val NotDefinedHereDenotation = new NoDenotation
+
   // ---- Completion --------------------------------------------------------
 
   /** Instances of LazyType are carried by uncompleted symbols.
diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index 512ddc41a..24f59ee43 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -1244,7 +1244,10 @@ object Types {
             }
           case d =>
             if (d.validFor.runId != ctx.period.runId) loadDenot
-            else d.current
+            else {
+              val newd = d.currentIfExists
+              if (newd ne NotDefinedHereDenotation) newd else loadDenot
+            }
         }
         if (ctx.typerState.ephemeral) record("ephemeral cache miss: loadDenot")
         else if (d.exists) {