enhance FinalCaseClasses rule to handle case classes defined inside traits and add corresponding test for SI-4440

Author: halotukozak

analyzer/src/main/scala/com/avsystem/commons/analyzer/FinalCaseClasses.scala

@@ -1,6 +1,8 @@
 package com.avsystem.commons
 package analyzer
 
+import com.avsystem.commons.analyzer.Level.Info
+
 import scala.tools.nsc.Global
 
 class FinalCaseClasses(g: Global) extends AnalyzerRule(g, "finalCaseClasses", Level.Warn) {
@@ -9,7 +11,15 @@ class FinalCaseClasses(g: Global) extends AnalyzerRule(g, "finalCaseClasses", Le
 
   def analyze(unit: CompilationUnit): Unit = unit.body.foreach {
     case cd: ClassDef if !cd.mods.hasFlag(Flag.FINAL) && cd.mods.hasFlag(Flag.CASE) =>
-      report(cd.pos, "Case classes should be marked as final")
+      // Skip case classes defined inside traits (SI-4440)
+
+      val isInner = cd.symbol.isStatic
+
+      if (isInner) {
+        report(cd.pos, "Case classes should be marked as final")
+      } else {
+        report(cd.pos, "Case classes should be marked as final. Due to the SI-4440 bug, it cannot be done here. Consider moving the case class to the companion object", level = Info)
+      }
     case _ =>
   }
-}
+}

analyzer/src/test/scala/com/avsystem/commons/analyzer/FinalCaseClassesTest.scala

@@ -36,4 +36,24 @@ class FinalCaseClassesTest extends AnyFunSuite with AnalyzerTest {
         |}
       """.stripMargin)
   }
+
+  // SI-4440 https://github.com/scala/bug/issues/4440
+  test("should not be affected due to SI-4440"){
+    assertNoErrors(
+      //language=scala
+      """
+        |trait Outer {
+        |  case class Inner(x: Int, y: String) {
+        |    def double: Int = x * 2
+        |  }
+        |}
+        |
+        |class Outer2 {
+        |  case class Inner(x: Int, y: String) {
+        |    def double: Int = x * 2
+        |  }
+        |}
+        """.stripMargin
+    )
+  }
 }