firestore/internal/conformance/testdata/update-arrayunion-noarray.json

{
  "tests": [
    {
      "description": "update: ArrayUnion cannot be in an array value",
      "comment": "ArrayUnion must be the value of a field. Firestore\ntransforms don't support array indexing.",
      "update": {
        "docRefPath": "projects/projectID/databases/(default)/documents/C/d",
        "jsonData": "{\"a\": [1, 2, [\"ArrayRemove\", 1, 2, 3]]}",
        "isError": true
      }
    }
  ]
}