The documentation for $all says:

The $all operator selects the documents where the value of a field is an array that contains all the specified elements. 

In the case of find({foo: {$all: []}}), there are no specified elements, which means that by this definition should match all documents where foo is an array (see SERVER-99070 for cases where the array clause is misleading). However, the current implementation will parse this query as $alwaysFalse.

To improve the situation, the implementation should either:

1. Parse to $alwaysTrue and update the docs to remove the array clause
2. Match all documents where foo is an array
3. Become a parse-time error similar to {$and: []}