Merge pull request #1020 from dotty-staging/fix/tvar-ephemeral

Fix caching bug: don't assume that tvars instantiation cannot be retracted

Author: odersky

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

@@ -2558,7 +2558,10 @@ object Types {
      *  uninstantiated
      */
     def instanceOpt(implicit ctx: Context): Type =
-      if (inst.exists) inst else ctx.typerState.instType(this)
+      if (inst.exists) inst else {
+        ctx.typerState.ephemeral = true
+        ctx.typerState.instType(this)
+      }
 
     /** Is the variable already instantiated? */
     def isInstantiated(implicit ctx: Context) = instanceOpt.exists