I noticed the following regression when trying to build an old F# project using the .NET 9 sdk. The following code

open System.Runtime.Serialization

type CartEvent =
    | [<DataMember(Name = "cartCreated")>] CartCreated

now fails to compile with the error

error FS0842: This attribute is not valid for use on this language element

Technically, this error is correct because DataMemberAttribute specifies AttributeTarget.Field ||| AttributeTarget.Property however this is breaking a common pattern used when serializing events modelled as discriminated unions.

So I have the following questions:

1. Was this change made intentionally rectifying lack of attribute target validation by the compiler?
2. What attribute AttributeTarget should library authors be specifying for attributes intended for application to union cases?

cc @bartelink