Scala 3: Add support for literal-based singleton types

[tarmath]

Add support for SIP-23 - Literal-based singleton types

play-json 3.0.4
JDK 17

Reproducible Test Case

package play.api.libs.json

object TestSingletonTypes {
  case class CaseClassWithSingleton(
    singleton: "singleton"
  )  

  object CaseClassWithSingleton {
    implicit val format: Format[CaseClassWithSingleton] = Json.format[CaseClassWithSingleton]
  }
}

Result

Fails to compile with:

[error] -- Error: /Users/mpichette/src/git/oss/play-json/play-json/shared/src/test/scala-3/play/api/libs/json/TestSingletonTypes.scala:13:60 
[error] 13 |    implicit val format: Format[CaseClassWithSingleton] = Json.format[CaseClassWithSingleton]
[error]    |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]    |                    Instance not found: play.api.libs.json.Reads[String]

[cchantep]

Hi,

I'm not sure I understand the use case for a case class field with a singleton type. By nature, I would expect a case class field to be assignable; otherwise, wouldn't it make more sense to use an invariant (constant)? Could you elaborate?