Fix caching logic in baseTypeRef

Typevars can be parts of larger types or underlying types of
other types, which renders these other types uncacheable. The new
logic takes account of that.

Author: odersky

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

@@ -1310,6 +1310,18 @@ object SymDenotations {
         case _ => bt
       }
 
+      def inCache(tp: Type) = baseTypeRefCache.containsKey(tp)
+
+      /** Can't cache types containing type variables which are uninstantiated
+       *  or whose instances can change, depending on typerstate.
+       */
+      def isCachable(tp: Type): Boolean = tp match {
+        case tp: TypeProxy => inCache(tp.underlying)
+        case tp: AndOrType => inCache(tp.tp1) && inCache(tp.tp2)
+        case tp: TypeVar => tp.inst.exists && inCache(tp.inst)
+        case _ => true
+      }
+
       def computeBaseTypeRefOf(tp: Type): Type = {
         Stats.record("computeBaseTypeOf")
         if (symbol.isStatic && tp.derivesFrom(symbol))
@@ -1326,9 +1338,6 @@ object SymDenotations {
               case _ =>
                 baseTypeRefOf(tp.underlying)
             }
-          case tp: TypeVar =>
-            if (tp.inst.exists) computeBaseTypeRefOf(tp.inst)
-            else Uncachable(computeBaseTypeRefOf(tp.underlying))
           case tp: TypeProxy =>
             baseTypeRefOf(tp.underlying)
           case AndType(tp1, tp2) =>
@@ -1349,14 +1358,9 @@ object SymDenotations {
             var basetp = baseTypeRefCache get tp
             if (basetp == null) {
               baseTypeRefCache.put(tp, NoPrefix)
-              basetp = computeBaseTypeRefOf(tp) match {
-                case Uncachable(basetp) =>
-                  baseTypeRefCache.remove(tp)
-                  basetp
-                case basetp =>
-                  baseTypeRefCache.put(tp, basetp)
-                  basetp
-              }
+              basetp = computeBaseTypeRefOf(tp)
+              if (isCachable(tp)) baseTypeRefCache.put(tp, basetp)
+              else baseTypeRefCache.remove(tp)
             } else if (basetp == NoPrefix) {
               throw CyclicReference(this)
             }
@@ -1438,8 +1442,6 @@ object SymDenotations {
       }
   }
 
-  private case class Uncachable(tp: Type) extends UncachedGroundType
-
   /** The denotation of a package class.
    *  It overrides ClassDenotation to take account of package objects when looking for members
    */