fix: emit right parent under for module classes in Scala 2

Logic ported from https://github.com/scala/scala/blob/6ecf74bfb4e914391fda1df5288ebaad6e4699b8/src/compiler/scala/tools/nsc/typechecker/Unapplies.scala#L122

Author: hamzaremmal

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

@@ -4,7 +4,7 @@ package ast
 
 import core.*
 import util.Spans.*, Types.*, Contexts.*, Constants.*, Names.*, NameOps.*, Flags.*
-import Symbols.*, StdNames.*, Trees.*, ContextOps.*
+import Symbols.*, StdNames.*, Definitions.*, Trees.*, ContextOps.*
 import Decorators.*
 import Annotations.Annotation
 import NameKinds.{UniqueName, ContextBoundParamName, ContextFunctionParamName, DefaultGetterName, WildcardParamName}
@@ -1024,7 +1024,12 @@ object desugar {
         val toStringMeth =
           DefDef(nme.toString_, Nil, TypeTree(), Literal(Constant(className.toString))).withMods(Modifiers(Override | Synthetic))
 
-        companionDefs(anyRef, applyMeths ::: unapplyMeth :: toStringMeth :: companionMembers)
+        if Feature.shouldBehaveAsScala2 && !mods.is(Abstract) && originalTparams.isEmpty && arity <= MaxImplementedFunctionArity then
+          val tpt = originalVparamss.head.map(_.tpt) :+ untpd.Ident(cdef.name)
+          val parent = appliedTypeTree(scalaRuntimeDot(s"AbstractFunction$arity".toTypeName), tpt)
+          companionDefs(parent, applyMeths ::: unapplyMeth :: toStringMeth :: companionMembers)
+        else
+          companionDefs(anyRef, applyMeths ::: unapplyMeth :: toStringMeth :: companionMembers)
       }
       else if (companionMembers.nonEmpty || companionDerived.nonEmpty || isEnum)
         companionDefs(anyRef, companionMembers)

project/MiMaFilters.scala

@@ -548,17 +548,6 @@ object MiMaFilters {
           ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.Tuple3Zipped.coll2$extension"),
           ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.Tuple3Zipped.coll3$extension"),
 
-          // singleton case classes modules inherit AbstractFunction1??
-          ProblemFilters.exclude[MissingTypesProblem]("scala.ScalaReflectionException$"),
-          ProblemFilters.exclude[DirectMissingMethodProblem]("scala.ScalaReflectionException.compose"),
-          ProblemFilters.exclude[DirectMissingMethodProblem]("scala.ScalaReflectionException.andThen"),
-          ProblemFilters.exclude[MissingTypesProblem]("scala.UninitializedFieldError$"),
-          ProblemFilters.exclude[DirectMissingMethodProblem]("scala.UninitializedFieldError.compose"),
-          ProblemFilters.exclude[DirectMissingMethodProblem]("scala.UninitializedFieldError.andThen"),
-          ProblemFilters.exclude[MissingTypesProblem]("scala.collection.StringView$"),
-          ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.StringView.compose"),
-          ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.StringView.andThen"),
-
           // TO INVESTIGATE: This constructor changed, but it is private... why complaining?
           ProblemFilters.exclude[IncompatibleMethTypeProblem]("scala.collection.immutable.LazyList.this"),
           // This one should be fine, public class inside private object