Shouldn't KeyTag be an abstract type rather than a trait?

[DmytroMitin]

type FieldType[K, +V] = V with KeyTag[K, V]
trait KeyTag[K, +V] 

trait Tagged[U]
type @@[+T, U] = T with Tagged[U]

None.asInstanceOf[FieldType[Symbol @@ "field1", Any]]
// ClassCastException: scala.None$ cannot be cast to KeyTag

vs.

type FieldType[K, +V] = V with KeyTag[K, V]
type KeyTag[K, +V] 

trait Tagged[U]
type @@[+T, U] = T with Tagged[U]

None.asInstanceOf[FieldType[Symbol @@ "field1", Any]] // no exception

Normally the trait works but not with Any or AnyRef.

field[Symbol @@ "field1"](None: Any) doesn't throw but val f = field[Symbol @@ "field1"](None: Any) does.

Also

def foo(x: Any): FieldType[Symbol @@ "field1", Any] = field[Symbol @@ "field1"](x)
foo(None)

throws.

Scala 2.13.10.

Discovered in https://stackoverflow.com/questions/74384745/encoding-decoding-a-field-with-any-datatype-assigned-with-value-none-in-scala

[DmytroMitin]

Or is it too late because of the binary compatibility?

[joroKr21]

I tried to make some of these types abstract a while ago - see #807 - one of the issues was it caused boxing which would be unacceptable performance regression. I don't think I tried to make the tag itself abstract though. Can you check if this would cause boxing? You can use :javap in the REPL to view the generated bytecode.

[joroKr21]

Or is it too late because of the binary compatibility?

That might be the case too. If MiMa complains then probably we can't do it.

[DmytroMitin]

#44 (noticed at https://stackoverflow.com/questions/75012811/should-tagged-types-work-for-value-classes )