Follow Scala 2 when generating trait constructors under -Ycompile-scala2-library (#22529)

Closes #22496

Author: hamzaremmal

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -466,6 +466,8 @@ trait UntypedTreeInfo extends TreeInfo[Untyped] { self: Trees.Instance[Untyped]
    */
   private def defKind(tree: Tree)(using Context): FlagSet = unsplice(tree) match {
     case EmptyTree | _: Import => NoInitsInterface
+    case tree: TypeDef if ctx.settings.YcompileScala2Library.value =>
+      if (tree.isClassDef) EmptyFlags else NoInitsInterface
     case tree: TypeDef => if (tree.isClassDef) NoInits else NoInitsInterface
     case tree: DefDef =>
       if tree.unforcedRhs == EmptyTree
@@ -477,6 +479,8 @@ trait UntypedTreeInfo extends TreeInfo[Untyped] { self: Trees.Instance[Untyped]
         NoInitsInterface
       else if tree.mods.is(Given) && tree.paramss.isEmpty then
         EmptyFlags // might become a lazy val: TODO: check whether we need to suppress NoInits once we have new lazy val impl
+      else if ctx.settings.YcompileScala2Library.value then
+        EmptyFlags
       else
         NoInits
     case tree: ValDef => if (tree.unforcedRhs == EmptyTree) NoInitsInterface else EmptyFlags

project/Scala2LibraryBootstrappedMiMaFilters.scala

@@ -15,9 +15,6 @@ object Scala2LibraryBootstrappedMiMaFilters {
         ProblemFilters.exclude[FinalClassProblem]("scala.language$experimental$"),
         ProblemFilters.exclude[FinalClassProblem]("scala.languageFeature$*$"),
 
-        // trait $init$
-        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.*.$init$"),
-
         // Value class extension methods
         ProblemFilters.exclude[DirectMissingMethodProblem]("scala.*$extension"),