Using Bson_serialization_type “any” means that idl compatibility checker script can’t do any type checking, since that logic is in cpp, so we want to disallow this type from parameters/reply fields as a result.

However, lots of commands actually use it for valid reasons.
For now we added some exceptions to allow using Bson_serialization_type “any” in our idl_checker script, these are the lisf of fields that still using Bson_serialization_type “any” that require your attention:

src/mongo/db/create_indexes.idl

1. The 'createIndexes'' command has parameter 'commitQuorum' that has type
2. 'CommitQuorum' 'createIndexes' has a reply field 'commitQuorum' of type 'CommitQuorum'  

 
Take a look at these fields and try to identify which ones can be converted away from using Bson_serialization_type “any”, and which ones must keep using it.
Here are the two outcomes:
1- if the field can be converted:

• This change should be done before 5.0 (if the field is "stable") as this will be the realese for API stable Version 1
• The exception for that field should be removed from idl_checker script (TODO will be added with this ticket number).

2- if the field can't be converted, or the change can't be done before 5.0.

• The type of that field type should stay of Bson_serialization_type “any” and can't be changed later for any reason unless it is defined as "unstable".
• We need to put comments on the serialization/deserialization to prevent the behavioral changes (preferably to add tests to ensure that.)