I can see your argument in some cases, but if you are accepting input, at some point you'll have to accept a string (or any primitive) and convert it to your custom type.
How does static typing ensure you're mapping the correct JSON input fields to your custom types?
> How does static typing ensure you're mapping the correct JSON input fields to your custom types?
Ideally you push it all the way out. Rather than JSON you use Thrift or similar where the messages are strongly typed.
But if you do use JSON, how much benefit does unit testing your JSON mapping really give you? The mapping in code is just a list of field:field pairs, the test is just the same thing, is there really any value in repeating it twice? I'd sooner rely on careful code review than a test.
How does static typing ensure you're mapping the correct JSON input fields to your custom types?