Refs #26709 -- Required a name for Indexes passed to AddIndex.

Thanks to Markush for discussions.

django/db/migrations/operations/models.py

@@ -751,6 +751,11 @@ class AddIndex(Operation):
 
     def __init__(self, model_name, index):
         self.model_name = model_name
+        if not index._name:
+            raise ValueError(
+                "Indexes passed to AddIndex operations require a name "
+                "argument. %r doesn't have one." % index
+            )
         self.index = index
 
     def state_forwards(self, app_label, state):