Load but not enter case accessors fields in Scala2Unpickler (#19926)

Fixes #19421

Author: sjrd

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -445,8 +445,6 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
        // Scala 2 sometimes pickle the same type parameter symbol multiple times
        // (see i11173 for an example), but we should only unpickle it once.
        || tag == TYPEsym && flags.is(TypeParam) && symScope(owner).lookup(name.asTypeName).exists
-       // We discard the private val representing a case accessor. We only load the case accessor def.
-       || flags.isAllOf(CaseAccessor| PrivateLocal, butNot = Method)
     then
       // skip this member
       return NoSymbol
@@ -534,7 +532,10 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
                 // parameter unpickling and try to emulate it.
                 !completer.areParamsInitialized
               case _ =>
-                true)
+                true) &&
+            // We discard the private val representing a case accessor. We only enter the case accessor def.
+            // We do need to load these symbols to read properly unpickle the annotations on the symbol (see sbt-test/scala2-compat/i19421).
+            !flags.isAllOf(CaseAccessor | PrivateLocal, butNot = Method)
 
         if (canEnter)
           owner.asClass.enter(sym, symScope(owner))

sbt-test/scala2-compat/i19421/S3SnapshotStoreSpec.scala

@@ -0,0 +1,8 @@
+import com.typesafe.config.Config
+import akka.persistence.snapshot.SnapshotStoreSpec
+
+object ConfigHelper {
+  def config(): Config = ???
+}
+
+class S3SnapshotStoreSpec extends SnapshotStoreSpec(null)

sbt-test/scala2-compat/i19421/build.sbt

@@ -0,0 +1,5 @@
+scalaVersion := sys.props("plugin.scalaVersion")
+
+libraryDependencies ++= Seq(
+  "com.typesafe.akka" %% "akka-persistence-tck" % "2.8.0"
+).map(_.cross(CrossVersion.for3Use2_13))

sbt-test/scala2-compat/i19421/test

@@ -0,0 +1,3 @@
+> compile
+> clean
+> compile