Attempt to make LVS compare output a little more predictable with boundary cases

- For unattached subcircuit pins no error should be reported
- For abstract nets, graph propagation through subcircuit pins isn't attempted.
  Abstract nets are only dummy-associated currently.

src/laybasic/laybasic/layNetlistBrowserModel.cc

@@ -1402,8 +1402,14 @@ static std::string search_string_from_names (const std::pair<const Obj *, const
 bool
 NetlistBrowserModel::is_valid_net_pair (const std::pair<const db::Net *, const db::Net *> &nets) const
 {
-  IndexedNetlistModel::circuit_pair net_parent = mp_indexer->parent_of (nets);
-  return (net_parent.first != 0 || net_parent.second != 0);
+  if (! nets.first && ! nets.second) {
+    //  this is a valid case: e.g. two matching subcircuit pins without nets attached
+    //  to them
+    return true;
+  } else {
+    IndexedNetlistModel::circuit_pair net_parent = mp_indexer->parent_of (nets);
+    return (net_parent.first != 0 || net_parent.second != 0);
+  }
 }
 
 db::NetlistCrossReference::Status

src/laybasic/laybasic/layNetlistCrossReferenceModel.cc

@@ -368,7 +368,9 @@ static size_t get_index_of (const Pair &pair, Iter begin, Iter end, std::map<Pai
     }
 
     i = cache.find (pair);
-    tl_assert (i != cache.end ());
+    if (i == cache.end ()) {
+      return lay::no_netlist_index;
+    }
 
   }