Compare the following:

structure Person {
  item: String = null
}

structure Person2 {
  item: String
}

structure Person3 {
  item: String = ""
}

Given a payload like {"item": null}, only Person2 will be decoded successfully if you're using generated code.

Noteworthy:

• Dynamic will accept the input in all cases, but only Person3 will have a field present with the default value. The others will simply not have that field.

Full repro: https://github.com/kubukoz/demos/tree/smithy4s-nulls-defaults (sbt run shows you all the results)

There's surely a bug here because Dynamic and Codegen don't work the same way, but which should it be? I'm leaning towards thinking that = null should be equivalent to nothing (see #1315), and I'm pretty sure all these cases should accept the input somehow.