Add test

compiler/src/dotty/tools/dotc/transform/init/CheckGlobal.scala

@@ -111,6 +111,10 @@ class CheckGlobal {
     else {
       val cdef = cls.defTree.asInstanceOf[TypeDef]
       val tpl = cdef.rhs.asInstanceOf[Template]
+
+      // ignore separately compiled classes
+      if !tpl.unforced.isInstanceOf[List[_]] then return Nil
+
       var dependencies: List[Dependency] = Nil
       val traverser = new TreeTraverser {
         override def traverse(tree: Tree)(using Context): Unit =

tests/init/neg/t5366.scala

@@ -0,0 +1,15 @@
+class IdAndMsg(val id: Int,  val msg: String = "")
+
+case object ObjA extends IdAndMsg(1)     // error
+case object ObjB extends IdAndMsg(2)     // error
+
+object IdAndMsg {                        // error
+  val values = List(ObjA , ObjB)
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    ObjA
+    println(IdAndMsg.values)
+  }
+}