Don't pickle OPAQUE for objects

Objects containing opaque aliases get the Opaque flag set for technical reasons.
But that should not leak into the Tasty format.

Author: odersky

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -638,11 +638,11 @@ class TreePickler(pickler: TastyPickler) {
     }
     if (flags.is(ParamAccessor) && sym.isTerm && !sym.isSetter)
       flags = flags &~ ParamAccessor // we only generate a tag for parameter setters
-    pickleFlags(flags, sym.isTerm)
+    pickleFlags(flags, sym)
     sym.annotations.foreach(pickleAnnotation(sym, _))
   }
 
-  def pickleFlags(flags: FlagSet, isTerm: Boolean)(implicit ctx: Context): Unit = {
+  def pickleFlags(flags: FlagSet, sym: Symbol)(implicit ctx: Context): Unit = {
     import Flags._
     def writeModTag(tag: Int) = {
       assert(isModifierTag(tag))
@@ -663,7 +663,7 @@ class TreePickler(pickler: TastyPickler) {
     if (flags.is(Synthetic)) writeModTag(SYNTHETIC)
     if (flags.is(Artifact)) writeModTag(ARTIFACT)
     if (flags.is(Scala2x)) writeModTag(SCALA2X)
-    if (isTerm) {
+    if (sym.isTerm) {
       if (flags.is(Implicit)) writeModTag(IMPLICIT)
       if (flags.is(Given)) writeModTag(GIVEN)
       if (flags.is(Erased)) writeModTag(ERASED)
@@ -685,7 +685,7 @@ class TreePickler(pickler: TastyPickler) {
       if (flags.is(Trait)) writeModTag(TRAIT)
       if (flags.is(Covariant)) writeModTag(COVARIANT)
       if (flags.is(Contravariant)) writeModTag(CONTRAVARIANT)
-      if (flags.is(Opaque)) writeModTag(OPAQUE)
+      if flags.is(Opaque) && sym.isOpaqueAlias then writeModTag(OPAQUE)
       if (flags.is(Open)) writeModTag(OPEN)
     }
   }