Currently, the validation within the ShardKeyPattern class doesn't check that the shard key fields don't have the "$" prefix. So the shardCollection command depends on the index key pattern validation here to prevent a shard key like {"$x": 1} from being used. However, "$**" is a special field name is allowed to bypass this check.

As a result, one can run a shardCollection command with the shard key with {"$**": 1} without getting any error. However, the resulting collection appears to have a number of unexpected behaviors. For example:

• Running a listCollections command would fail with a BadValue error "unknown top level operator: $**. If you have a field name that starts with a '$' symbol, consider using $getField or $setField.".
• Running a {$collStats: {}} aggregation would hit this tassert in QueryPlannerIXSelect.