Mention fullname of PlaceholderNode in assertion (#10565)

Helps with #10542

Co-authored-by: Shantanu <[email protected]>
Co-authored-by: hauntsaninja <>

Author: TheCleric

mypy/nodes.py

@@ -3080,7 +3080,9 @@ def serialize(self, prefix: str, name: str) -> JsonDict:
                         and fullname != prefix + '.' + name
                         and not (isinstance(self.node, Var)
                                  and self.node.from_module_getattr)):
-                    assert not isinstance(self.node, PlaceholderNode)
+                    assert not isinstance(self.node, PlaceholderNode), (
+                        'Definition of {} is unexpectedly incomplete'.format(fullname)
+                    )
                     data['cross_ref'] = fullname
                     return data
             data['node'] = self.node.serialize()