Why compiler assuming `json['x'] = int?`?

Because the [] operator on Map returns a nullable by spec:

V? operator [](Object? key)

The value for the given key, or null if key is not in the map.


So if you are asking for a key that is not in your Map you will get a null value back and not an exception.

If you are 100% sure json['x'] will always work and want the application to crash in case this is not the case, you can use json['x']!. Alternative, you need to provide default values or other type of handling in case these values is not in the map.