Make definitions used in capture checking again @experimental

#16199 dropped their experimental status to allow experimenting
with compiling the compiler under -Ycc, but that was meant as a
local change that should not have been propagated to main.

Author: odersky

library/src/scala/annotation/retains.scala

@@ -11,8 +11,8 @@ package scala.annotation
  *  The annotation can also be written explicitly if one wants to avoid the
  *  non-standard capturing type syntax.
  */
-// @experimental // suppressed so we can use in compiler
+@experimental
 class retains(xs: Any*) extends annotation.StaticAnnotation
 
-// @experimental // suppressed so we can use in compiler
+@experimental
 class retainsUniversal extends annotation.StaticAnnotation

library/src/scala/caps.scala

@@ -2,7 +2,7 @@ package scala
 
 import annotation.experimental
 
-// @experimental , suppress @experimental so we can use in compiler itself
+@experimental
 object caps:
 
   /** If argument is of type `cs T`, converts to type `box cs T`. This

project/MiMaFilters.scala

@@ -4,9 +4,5 @@ import com.typesafe.tools.mima.core._
 object MiMaFilters {
   val Library: Seq[ProblemFilter] = Seq(
     ProblemFilters.exclude[MissingClassProblem]("scala.annotation.internal.MappedAlternative"),
-    ProblemFilters.exclude[MissingClassProblem]("scala.caps"),
-    ProblemFilters.exclude[MissingClassProblem]("scala.caps$"),
-    ProblemFilters.exclude[MissingClassProblem]("scala.annotation.retains"),
-    ProblemFilters.exclude[MissingClassProblem]("scala.annotation.retainsUniversal"),
   )
 }

tests/run-custom-args/tasty-inspector/stdlibExperimentalDefinitions.scala

@@ -50,8 +50,11 @@ val experimentalDefinitionInLibrary = Set(
   "scala.annotation.capability",
   "scala.annotation.internal.CaptureChecked",
   "scala.annotation.internal.requiresCapability",
-  //"scala.annotation.retains",
+  "scala.annotation.retains",
+  "scala.annotation.retainsUniversal",
   "scala.annotation.retainsByName",
+  "scala.caps",
+  "scala.caps$",
 
   //// New APIs: Mirror
   // Can be stabilized in 3.3.0 or later.