Check for duplicate fields in the declaration visitor

boot/symbol.cc

@@ -22,7 +22,6 @@ namespace elna
 namespace boot
 {
     type::type()
-        : tag(type_tag::alias)
     {
     }
 
@@ -145,6 +144,7 @@ namespace boot
         case type_tag::empty:
             break;
         case type_tag::alias:
+            this->alias.~weak_ptr<alias_type>();
             break;
         case type_tag::primitive:
             this->primitive.~shared_ptr<primitive_type>();