When using CreateIndex users are forced to use CreateIndex<BsonDocument> and then specify their partial filters using the bson syntax such as:

mb.Entity<Customer()
  .HasIndex(a => a.PostCode)
  .HasCreateIndexOptions(new CreateIndexOptions<BsonDocument>(
     PartialFilterExpression = Builders<BsonDocument>.Filter.Eq(a => a["Country"], "UK"));

Ideally they would be able to specify it using the correct type not BsonDocument to allow us to use EF's element name mapping and compile-time checking.

Reported on the forums at https://www.mongodb.com/community/forums/t/ef-provider-support-for-hasindex-on-nested-objects/310463/5