Add diagnostic why a symbol is stale.

Author: odersky

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

@@ -590,7 +590,10 @@ object Denotations {
         } while (d ne denot)
         this
       case _ =>
-        if (coveredInterval.containsPhaseId(ctx.phaseId)) staleSymbolError
+        if (coveredInterval.containsPhaseId(ctx.phaseId)) {
+          if (ctx.settings.debug.value) ctx.traceInvalid(this)
+          staleSymbolError
+        }
         else NoDenotation
     }
 

src/dotty/tools/dotc/core/SymDenotations.scala

@@ -57,7 +57,37 @@ trait SymDenotations { this: Context =>
       } catch {
         case ex: StaleSymbol => false
       }
+
+  /** Explain why symbol is invalid; used for debugging only */
+  def traceInvalid(denot: Denotation): Boolean = {
+    def show(d: Denotation) = s"$d#${d.symbol.id}"
+    def explain(msg: String) = {
+      println(s"${show(denot)} is invalid at ${this.period} because $msg")
+      false
+    }
+    denot match {
+      case denot: SymDenotation =>
+        def explainSym(msg: String) = explain(s"$msg\n defined = ${denot.definedPeriodsString}")
+        if (denot.is(ValidForever) || denot.isRefinementClass) true
+        else {
+          implicit val ctx: Context = this
+          val initial = denot.initial
+          if ((initial ne denot) || ctx.phaseId != initial.validFor.firstPhaseId) {
+            ctx.withPhase(initial.validFor.firstPhaseId).traceInvalid(initial.asSymDenotation)
+          } else try {
+            val owner = denot.owner.denot
+            if (!traceInvalid(owner)) explainSym("owner is invalid")
+            else if (!owner.isClass || owner.isRefinementClass || denot.isSelfSym) true
+            else if (owner.unforcedDecls.lookupAll(denot.name) contains denot.symbol) true
+            else explainSym(s"decls of ${show(owner)} are ${owner.unforcedDecls.lookupAll(denot.name).toList}, do not contain ${denot.symbol}")
+        } catch {
+          case ex: StaleSymbol => explainSym(s"$ex was thrown")
+        }
+      }
+      case _ =>
+        explain("denotation is not a SymDenotation")
     }
+  }
 }
 
 object SymDenotations {