Call normalizeOpaque in window of vulnerability

It turns out we get confused when we call normalizeOpaque as part of normal
completion during computeDenot. Fine tuning normalizeOpaque and calling it
in the window of vulnerability where a type is Opaque but not yet Deferred
fixes the problem. Before the following tests failed when compiled from-tasty:

    tests/run/implicit-specifity.scala failed
    tests/pos/i5720.scala failed
    tests/pos/toplevel-opaque-xm failed
    tests/pos/postconditions.scala failed

Author: odersky

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

@@ -384,8 +384,6 @@ object SymDenotations {
       if (isOpaqueAlias) {
         info match {
           case TypeAlias(alias) =>
-            info = TypeBounds(defn.NothingType, abstractRHS(alias))
-
             def refineSelfType(selfType: Type) =
               RefinedType(selfType, name, TypeAlias(alias))
             val enclClassInfo = owner.asClass.classInfo
@@ -395,8 +393,9 @@ object SymDenotations {
               case self: Symbol =>
                 self.info = refineSelfType(self.info)
             }
-
+            info = TypeBounds(defn.NothingType, abstractRHS(alias))
             setFlag(Deferred)
+            typeRef.recomputeDenot()
           case _ =>
         }
       }

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

@@ -1884,8 +1884,10 @@ object Types {
             finish(memberDenot(symd.initial.name, allowPrivate = false))
           else if (prefix.isArgPrefixOf(symd))
             finish(argDenot(sym.asType))
-          else if (infoDependsOnPrefix(symd, prefix))
+          else if (infoDependsOnPrefix(symd, prefix)) {
+            if (!symd.isClass && symd.is(Opaque, butNot = Deferred)) symd.normalizeOpaque()
             finish(memberDenot(symd.initial.name, allowPrivate = symd.is(Private)))
+          }
           else
             finish(symd.current)
       }