Make Parser covariant in Scala 3

core/shared/src/main/scala-2/caseapp/core/parser/Internal.scala

@@ -0,0 +1,5 @@
+package caseapp.core.parser
+
+private[parser] object Internal {
+  type uncheckedVarianceScala2 = scala.annotation.unchecked.uncheckedVariance
+}

core/shared/src/main/scala-3/caseapp/core/parser/Internal.scala

@@ -0,0 +1,5 @@
+package caseapp.core.parser
+
+private[parser] object Internal {
+  final class uncheckedVarianceScala2 extends scala.annotation.StaticAnnotation
+}

core/shared/src/main/scala-3/caseapp/core/parser/Parser.scala

@@ -12,7 +12,7 @@ import scala.language.implicitConversions
   * @tparam T:
   *   success result type
   */
-abstract class Parser[T] extends ParserMethods[T] {
+abstract class Parser[+T] extends ParserMethods[T] {
 
   import Parser.Step
 

core/shared/src/main/scala/caseapp/core/complete/Completer.scala

@@ -3,7 +3,7 @@ package caseapp.core.complete
 import caseapp.core.Arg
 import caseapp.core.help.{WithFullHelp, WithHelp}
 
-trait Completer[T] { self =>
+trait Completer[-T] { self =>
   def optionName(prefix: String, state: Option[T]): List[CompletionItem]
   def optionValue(arg: Arg, prefix: String, state: Option[T]): List[CompletionItem]
   def argument(prefix: String, state: Option[T]): List[CompletionItem]

core/shared/src/main/scala/caseapp/core/help/WithFullHelp.scala

@@ -4,7 +4,7 @@ import caseapp.core.Error
 import caseapp.{ExtraName, Group, HelpMessage, Recurse}
 import caseapp.core.parser.Parser
 
-final case class WithFullHelp[T](
+final case class WithFullHelp[+T](
   @Recurse
   withHelp: WithHelp[T],
   @Group("Help")

core/shared/src/main/scala/caseapp/core/help/WithHelp.scala

@@ -14,7 +14,7 @@ import caseapp.core.Error
   * @tparam T:
   *   type to which usage and help options are added
   */
-final case class WithHelp[T](
+final case class WithHelp[+T](
   @Group("Help")
   @HelpMessage("Print usage and exit")
   usage: Boolean = false,

core/shared/src/main/scala/caseapp/core/parser/ParserMethods.scala

@@ -6,9 +6,10 @@ import caseapp.core.util.Formatter
 import caseapp.Name
 import caseapp.core.complete.Completer
 import caseapp.core.complete.CompletionItem
+
 import scala.annotation.tailrec
 
-trait ParserMethods[T] { parser: Parser[T] =>
+trait ParserMethods[+T] { parser: Parser[T @Internal.uncheckedVarianceScala2] =>
 
   import Parser.Step