3 files changed, 12 insertions, 1 deletions
diff --git a/src/compiler/scala/tools/nsc/typechecker/Namers.scala b/src/compiler/scala/tools/nsc/typechecker/Namers.scala
index 48c2c55d7c..fd093614b5 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Namers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Namers.scala
@@ -148,6 +148,13 @@ trait Namers extends MethodSynthesis {
     def updatePosFlags(sym: Symbol, pos: Position, flags: Long): Symbol = {
       debuglog("[overwrite] " + sym)
       val newFlags = (sym.flags & LOCKED) | flags
+      sym.rawInfo match {
+        case tr: TypeRef =>
+          // !!! needed for: pos/t5954d; the uniques type cache will happilly serve up the same TypeRef
+          // over this mutated symbol, and we witness a stale cache for `parents`.
+          tr.invalidateCaches()
+        case _ =>
+      }
       sym reset NoType setFlag newFlags setPos pos
       sym.moduleClass andAlso (updatePosFlags(_, pos, moduleClassFlags(flags)))
 
diff --git a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
index 95f2620061..f5893172e8 100644
--- a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
@@ -857,7 +857,7 @@ abstract class RefChecks extends InfoTransform with scala.reflect.internal.trans
         val baseClass = clazz.info.baseTypeSeq(i).typeSymbol
         seenTypes(i) match {
           case Nil =>
-            println("??? base "+baseClass+" not found in basetypes of "+clazz)
+            devWarning(s"base $baseClass not found in basetypes of $clazz. This might indicate incorrect caching of TypeRef#parents.")
           case _ :: Nil =>
             ;// OK
           case tp1 :: tp2 :: _ =>
diff --git a/src/reflect/scala/reflect/internal/Types.scala b/src/reflect/scala/reflect/internal/Types.scala
index 99e6ae633f..437b962abf 100644
--- a/src/reflect/scala/reflect/internal/Types.scala
+++ b/src/reflect/scala/reflect/internal/Types.scala
@@ -2218,6 +2218,10 @@ trait Types
         trivial = fromBoolean(!sym.isTypeParameter && pre.isTrivial && areTrivialTypes(args))
       toBoolean(trivial)
     }
+    private[scala] def invalidateCaches(): Unit = {
+      parentsPeriod = NoPeriod
+      baseTypeSeqPeriod = NoPeriod
+    }
     private[reflect] var parentsCache: List[Type]      = _
     private[reflect] var parentsPeriod                 = NoPeriod
     private[reflect] var baseTypeSeqCache: BaseTypeSeq = _