4 files changed, 22 insertions, 7 deletions
diff --git a/src/dotty/tools/dotc/core/Substituters.scala b/src/dotty/tools/dotc/core/Substituters.scala
index 74388f436..e05f76cd9 100644
--- a/src/dotty/tools/dotc/core/Substituters.scala
+++ b/src/dotty/tools/dotc/core/Substituters.scala
@@ -145,7 +145,7 @@ trait Substituters { this: Context =>
         var fs = from
         var ts = to
         while (fs.nonEmpty) {
-          if (fs.head eq sym) return ThisType(ts.head.asClass)
+          if (fs.head eq sym) return ts.head.asClass.thisType
           fs = fs.tail
           ts = ts.tail
         }
diff --git a/src/dotty/tools/dotc/core/SymDenotations.scala b/src/dotty/tools/dotc/core/SymDenotations.scala
index a3828552d..cb457bd73 100644
--- a/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -1011,7 +1011,7 @@ object SymDenotations {
         if ((pre eq NoPrefix) || ctx.erasedTypes) pre select sourceModule
         else TermRef.withSig(pre, name.sourceModuleName, Signature.NotAMethod)
       }
-      else ThisType(classSymbol)
+      else ThisType.raw(classSymbol)
 
     private[this] var myTypeRef: TypeRef = null
 
diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index a6ba7d9a0..787c7c4e9 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -1535,16 +1535,31 @@ object Types {
   // --- Other SingletonTypes: ThisType/SuperType/ConstantType ---------------------------
 
   /** The type cls.this */
-  abstract case class ThisType(cls: ClassSymbol) extends CachedProxyType with SingletonType {
-    override def underlying(implicit ctx: Context) = cls.classInfo.selfType
-    override def computeHash = doHash(cls)
+  abstract case class ThisType(private var myCls: ClassSymbol) extends CachedProxyType with SingletonType {
+    def cls(implicit ctx: Context): ClassSymbol =
+      try myCls.denot.symbol.asClass
+      catch {
+        case ex: StaleSymbol =>
+          def prevDenot(implicit ctx: Context) = myCls.denot
+          val prev = prevDenot(ctx.fresh.setPeriod(Period(myCls.defRunId, ctx.phaseId)))
+          myCls = prev.owner.info.member(prev.name).symbol.asClass
+          cls
+      }
+    override def underlying(implicit ctx: Context) =
+      try cls.classInfo.selfType
+      catch {
+        case ex: StaleSymbol =>
+          println(i"stale symbol when deref ${cls.id}")
+          throw ex
+      }
+    override def computeHash = doHash(myCls)
   }
 
   final class CachedThisType(cls: ClassSymbol) extends ThisType(cls)
 
   // TODO: consider hash before constructing types?
   object ThisType {
-    def apply(cls: ClassSymbol)(implicit ctx: Context) =
+    def raw(cls: ClassSymbol)(implicit ctx: Context) =
       unique(new CachedThisType(cls))
   }
 
diff --git a/src/dotty/tools/dotc/core/pickling/UnPickler.scala b/src/dotty/tools/dotc/core/pickling/UnPickler.scala
index 2e21358e4..e82002c9f 100644
--- a/src/dotty/tools/dotc/core/pickling/UnPickler.scala
+++ b/src/dotty/tools/dotc/core/pickling/UnPickler.scala
@@ -613,7 +613,7 @@ class UnPickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClassRoot:
       case THIStpe =>
         val cls = readSymbolRef().asClass
         if (isRefinementClass(cls)) RefinedThis(refinementTypes(cls))
-        else ThisType(cls)
+        else cls.thisType
       case SINGLEtpe =>
         val pre = readTypeRef()
         val sym = readDisambiguatedSymbolRef(_.info.isParameterless)