Temporary ad-hoc fix for scala#3971.

Author: gsps

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -396,7 +396,8 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
   private def refPurity(tree: Tree)(implicit ctx: Context): PurityLevel =
     if (!tree.tpe.widen.isParameterless) Pure
     else if (!tree.symbol.isStable) Impure
-    else if (tree.symbol.is(Lazy)) Idempotent // TODO add Module flag, sinxce Module vals or not Lazy from the start.
+    else if (tree.symbol.is(Lazy) && !defn.ScalaValueClasses().contains(tree.symbol.companionClass))
+      Idempotent // TODO add Module flag, sinxce Module vals or not Lazy from the start.
     else Pure
 
   def isPureRef(tree: Tree)(implicit ctx: Context) =