Kotlin JPA entity ID

You can use a 0 value rather than a null value.

@Entity
data class User (
        @Id @GeneratedValue
        var id: Long = 0,
        ...
)

Autogeneration should still find the next sequence.

Kotlin compiles to Java, which has both, a primitive type long and a Class Long

As per the Java Persistence Specification in section 11.1.21 Id Annotation both can be used for the Id:

The field or property to which the Id annotation is applied should be one of the following types: any Java primitive type; any primitive wrapper type; java.lang.String; java.util.Date; java.sql.Date; java.math.BigDecimal; java.math.BigInteger[109].

There is an advantage in using the Class over the primitive, as null has a more unambiguous meaning. But from the spec both are possible and you have to decide weather you favor Kotlins nullsafety over the the jpa style or the other way around.