Emit all classes as public to avoid object deserialization issues

This aligns our behavior with Scala 2 and fixes the issue encountered in
typelevel/cats-effect#2360 (comment).

Alternatively, we could change ModuleSerializationProxy upstream to call
`setAccessible(true)` on the MODULE$ field, but this wouldn't work if the object
in question is inside a Java 9+ module.

compiler/src/dotty/tools/backend/jvm/BTypesFromSymbols.scala

@@ -296,7 +296,11 @@ class BTypesFromSymbols[I <: DottyBackendInterface](val int: I) extends BTypes {
    */
   final def javaFlags(sym: Symbol): Int = {
 
-    val privateFlag = sym.is(Private) || (sym.isPrimaryConstructor && sym.owner.isTopLevelModuleClass)
+    // Classes are always emitted as public. This matches the behavior of Scala 2
+    // and is necessary for object deserialization to work properly, otherwise
+    // ModuleSerializationProxy may fail with an accessiblity error (see
+    // tests/run/serialize.scala and https://github.com/typelevel/cats-effect/pull/2360).
+    val privateFlag = !sym.isClass && (sym.is(Private) || (sym.isPrimaryConstructor && sym.owner.isTopLevelModuleClass))
 
     val finalFlag = sym.is(Final) && !toDenot(sym).isClassConstructor && !sym.is(Mutable) && !sym.enclosingClass.is(Trait)
 

project/MiMaFilters.scala

@@ -21,6 +21,11 @@ object MiMaFilters {
     ProblemFilters.exclude[DirectMissingMethodProblem]("scala.deriving.Mirror.fromTuple"),
 
     // Private to the compiler - needed for forward binary compatibility
-    ProblemFilters.exclude[MissingClassProblem]("scala.annotation.since")
+    ProblemFilters.exclude[MissingClassProblem]("scala.annotation.since"),
+
+    // Private inner classes, but we emit all classes as public in Java bytecode
+    ProblemFilters.exclude[InaccessibleClassProblem]("scala.quoted.FromExpr$PrimitiveFromExpr"),
+    ProblemFilters.exclude[InaccessibleClassProblem]("scala.quoted.Type$ValueOf$"),
+    ProblemFilters.exclude[InaccessibleClassProblem]("scala.reflect.Selectable$DefaultSelectable"),
   )
 }

tests/run/i4404a.check

@@ -1,8 +1,4 @@
-false
-false
-false
-true
-false
-true
-true
-true
+public class Car
+public final class Volvo
+public final class Car$
+public static final class Car$$anon$1

tests/run/i4404a.scala

@@ -8,12 +8,10 @@ object Car {
 }
 
 object Test {
-  def main(args: Array[String]) = {
-    List(new Car, new Volvo, Car, Car.car)
-      .map(_.getClass.getModifiers)
-      .foreach { m =>
-        println(Modifier.isPrivate(m))
-        println(Modifier.isFinal(m))
-      }
+  def main(args: Array[String]): Unit = {
+    val l =  List(new Car, new Volvo, Car, Car.car)
+      .map(_.getClass)
+      .map(cls => s"${Modifier.toString(cls.getModifiers)} $cls")
+    println(l.mkString("\n"))
   }
 }

tests/run/i4404b.check

@@ -1,2 +1,2 @@
-true
+false
 true

tests/run/serialize.scala

@@ -1,3 +1,14 @@
+package a {
+  object Outer extends Serializable {
+    private object Inner extends Serializable
+
+    val inner: AnyRef = Inner
+  }
+  class Bar extends Serializable {
+    val x: AnyRef = Outer.inner
+  }
+}
+
 object Test {
   def serializeDeserialize[T <: AnyRef](obj: T): T = {
     import java.io.*
@@ -26,5 +37,9 @@ object Test {
 
     val baz = serializeDeserialize(Baz)
     assert(baz ne Baz)
+
+    val bar = new a.Bar
+    val bar1 = serializeDeserialize(bar)
+    assert(bar.x eq bar1.x)
   }
 }