where T : record

[TonyValenti]

What is the right way to specify this method?

    internal static class WithExtensions {
        public static T WithBase<T>(this T This, ImportJobBase Base)
            where T : record, ICsvBase {

            var ret = This with {
                SourceId = Base.Source_Id,
                ResultId = Base.Result_Id,
                Created_At = Base.Dest_Created_At,
                Created_By = Base.Dest_Created_By,
                Updated_At = Base.Dest_Updated_At,
                Updated_By = Base.Dest_Updated_By,
            };

            return ret;
        }
    }

[HaloFour]

See: #3859

[huoyaoyuan]

Currently there should be no way.
In future, specifying the Clone method and init accessors in the interface may work.

[jnm2]

If a library can tell the difference between classes I compiled using the record keyword and classes I compiled using the class keyword, that threatens my ability to be able to treat record vs class as an implementation detail that won't affect libraries.

I see the ask, though, which is less "can I tell if this is a record" and more "can I use withers on this thing." That's different and would not involve where T : record. The title of this discussion is going to skew the conversation in terms of my first point.