Emit mixin forwarders as ordinary, non-bridge methods again (#21890)

Forward port of scala/scala#8037

Reverts #6352 and #6141.

Fixes #19270.

Author: hamzaremmal

.gitmodules

@@ -100,6 +100,7 @@
 [submodule "community-build/community-projects/scala-parallel-collections"]
 	path = community-build/community-projects/scala-parallel-collections
 	url = https://github.com/dotty-staging/scala-parallel-collections.git
+	branch = serialisation-stability-fix
 [submodule "community-build/community-projects/scala-collection-compat"]
 	path = community-build/community-projects/scala-collection-compat
 	url = https://github.com/dotty-staging/scala-collection-compat.git

community-build/community-projects/scala-parallel-collections

@@ -31,6 +31,7 @@ import dotty.tools.dotc.core.Types.*
 import dotty.tools.dotc.core.TypeErasure
 import dotty.tools.dotc.transform.GenericSignatures
 import dotty.tools.dotc.transform.ElimErasedValueType
+import dotty.tools.dotc.transform.Mixin
 import dotty.tools.io.AbstractFile
 import dotty.tools.dotc.report
 
@@ -395,12 +396,20 @@ trait BCodeHelpers extends BCodeIdiomatic {
      */
     def getGenericSignature(sym: Symbol, owner: Symbol): String = {
       atPhase(erasurePhase) {
-        val memberTpe =
+        def computeMemberTpe(): Type =
           if (sym.is(Method)) sym.denot.info
           else if sym.denot.validFor.phaseId > erasurePhase.id && sym.isField && sym.getter.exists then
             // Memoization field of getter entered after erasure, see run/i17069 for an example
             sym.getter.denot.info.resultType
           else owner.denot.thisType.memberInfo(sym)
+
+        val memberTpe = if sym.is(MixedIn) then
+          mixinPhase.asInstanceOf[Mixin].mixinForwarderGenericInfos.get(sym) match
+            case Some(genericInfo) => genericInfo
+            case none              => computeMemberTpe()
+        else
+          computeMemberTpe()
+
         getGenericSignatureHelper(sym, owner, memberTpe).orNull
       }
     }
@@ -491,7 +500,7 @@ trait BCodeHelpers extends BCodeIdiomatic {
       report.debuglog(s"Potentially conflicting names for forwarders: $conflictingNames")
 
       for (m0 <- sortedMembersBasedOnFlags(moduleClass.info, required = Method, excluded = ExcludedForwarder)) {
-        val m = if (m0.is(Bridge)) m0.nextOverriddenSymbol else m0
+        val m = if (m0.isOneOf(Bridge | MixedIn)) m0.nextOverriddenSymbol else m0
         if (m == NoSymbol)
           report.log(s"$m0 is a bridge method that overrides nothing, something went wrong in a previous phase.")
         else if (m.isType || m.is(Deferred) || (m.owner eq defn.ObjectClass) || m.isConstructor || m.name.is(ExpandedName))
@@ -507,10 +516,7 @@ trait BCodeHelpers extends BCodeIdiomatic {
           // we generate ACC_SYNTHETIC forwarders so Java compilers ignore them.
           val isSynthetic =
             m0.name.is(NameKinds.SyntheticSetterName) ||
-            // Only hide bridges generated at Erasure, mixin forwarders are also
-            // marked as bridge but shouldn't be hidden since they don't have a
-            // non-bridge overload.
-            m0.is(Bridge) && m0.initial.validFor.firstPhaseId == erasurePhase.next.id
+            m0.is(Bridge)
           addForwarder(jclass, moduleClass, m, isSynthetic)
         }
       }

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

@@ -299,7 +299,7 @@ class BTypesFromSymbols[I <: DottyBackendInterface](val int: I, val frontendAcce
         // illegal combination of modifiers at the bytecode level so
         // suppress final if abstract if present.
         && !sym.isOneOf(AbstractOrTrait)
-        //  Mixin forwarders are bridges and can be final, but final bridges confuse some frameworks
+        // Bridges can be final, but final bridges confuse some frameworks
         && !sym.is(Bridge), ACC_FINAL)
       .addFlagIf(sym.isStaticMember, ACC_STATIC)
       .addFlagIf(sym.is(Bridge), ACC_BRIDGE | ACC_SYNTHETIC)

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

@@ -239,6 +239,7 @@ object Phases {
     private var myErasurePhase: Phase = uninitialized
     private var myElimErasedValueTypePhase: Phase = uninitialized
     private var myLambdaLiftPhase: Phase = uninitialized
+    private var myMixinPhase: Phase = uninitialized
     private var myCountOuterAccessesPhase: Phase = uninitialized
     private var myFlattenPhase: Phase = uninitialized
     private var myGenBCodePhase: Phase = uninitialized
@@ -266,6 +267,7 @@ object Phases {
     final def gettersPhase: Phase = myGettersPhase
     final def erasurePhase: Phase = myErasurePhase
     final def elimErasedValueTypePhase: Phase = myElimErasedValueTypePhase
+    final def mixinPhase: Phase = myMixinPhase
     final def lambdaLiftPhase: Phase = myLambdaLiftPhase
     final def countOuterAccessesPhase = myCountOuterAccessesPhase
     final def flattenPhase: Phase = myFlattenPhase
@@ -295,6 +297,7 @@ object Phases {
       myErasurePhase = phaseOfClass(classOf[Erasure])
       myElimErasedValueTypePhase = phaseOfClass(classOf[ElimErasedValueType])
       myPatmatPhase = phaseOfClass(classOf[PatternMatcher])
+      myMixinPhase = phaseOfClass(classOf[Mixin])
       myLambdaLiftPhase = phaseOfClass(classOf[LambdaLift])
       myCountOuterAccessesPhase = phaseOfClass(classOf[CountOuterAccesses])
       myFlattenPhase = phaseOfClass(classOf[Flatten])
@@ -550,6 +553,7 @@ object Phases {
   def gettersPhase(using Context): Phase                = ctx.base.gettersPhase
   def erasurePhase(using Context): Phase                = ctx.base.erasurePhase
   def elimErasedValueTypePhase(using Context): Phase    = ctx.base.elimErasedValueTypePhase
+  def mixinPhase(using Context): Phase                  = ctx.base.mixinPhase
   def lambdaLiftPhase(using Context): Phase             = ctx.base.lambdaLiftPhase
   def flattenPhase(using Context): Phase                = ctx.base.flattenPhase
   def genBCodePhase(using Context): Phase               = ctx.base.genBCodePhase

compiler/src/dotty/tools/dotc/core/Phases.scala

@@ -16,6 +16,7 @@ import StdNames.*
 import Names.*
 import NameKinds.*
 import NameOps.*
+import Phases.erasurePhase
 import ast.Trees.*
 
 import dotty.tools.dotc.transform.sjs.JSSymUtils.isJSType
@@ -115,6 +116,15 @@ object Mixin {
 class Mixin extends MiniPhase with SymTransformer { thisPhase =>
   import ast.tpd.*
 
+  /** Infos before erasure of the generated mixin forwarders.
+   *
+   *  These will be used to generate Java generic signatures of the mixin
+   *  forwarders. Normally we use the types before erasure; we cannot do that
+   *  for mixin forwarders since they are created after erasure, and therefore
+   *  their type history does not have anything recorded for before erasure.
+   */
+  val mixinForwarderGenericInfos = MutableSymbolMap[Type]()
+
   override def phaseName: String = Mixin.name
 
   override def description: String = Mixin.description
@@ -305,8 +315,25 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
       for (meth <- mixin.info.decls.toList if needsMixinForwarder(meth))
       yield {
         util.Stats.record("mixin forwarders")
-        transformFollowing(DefDef(mkForwarderSym(meth.asTerm, Bridge), forwarderRhsFn(meth)))
+        transformFollowing(DefDef(mkMixinForwarderSym(meth.asTerm), forwarderRhsFn(meth)))
+      }
+
+    def mkMixinForwarderSym(target: TermSymbol): TermSymbol =
+      val sym = mkForwarderSym(target, extraFlags = MixedIn)
+      val (infoBeforeErasure, isDifferentThanInfoNow) = atPhase(erasurePhase) {
+        val beforeErasure = cls.thisType.memberInfo(target)
+        (beforeErasure, !(beforeErasure =:= sym.info))
       }
+      if isDifferentThanInfoNow then
+        // The info before erasure would not have been the same as the info now.
+        // We want to store it for the backend to compute the generic Java signature.
+        // However, we must still avoid doing that if erasing that signature would
+        // not give the same erased type. If it doesn't, we'll just give a completely
+        // incorrect Java signature. (This could be improved by generating dedicated
+        // bridges, but we don't go that far; scalac doesn't either.)
+        if TypeErasure.transformInfo(target, infoBeforeErasure) =:= sym.info then
+          mixinForwarderGenericInfos(sym) = infoBeforeErasure
+      sym
 
     cpy.Template(impl)(
       constr =

compiler/src/dotty/tools/dotc/transform/Mixin.scala

@@ -12,13 +12,11 @@ i9473.scala
 i13433.scala
 i13433b.scala
 macros-in-same-project1
-mixin-forwarder-overload
 t10889
 t3452d
 t3452e
 t3452g
 t7374
-t8905
 tuple-drop.scala
 tuple-ops.scala
 tuple-ops.scala

compiler/test/dotc/run-test-pickling.excludelist

@@ -193,7 +193,45 @@ object Scala2LibraryBootstrappedMiMaFilters {
         "scala.util.matching.Regex.Groups", "scala.util.matching.Regex.Match",
         "scala.util.package.chaining",
         "scala.util.Using.Manager", "scala.util.Using.Releasable", "scala.util.Using#Releasable.AutoCloseableIsReleasable",
-      ).map(ProblemFilters.exclude[MissingFieldProblem])
+      ).map(ProblemFilters.exclude[MissingFieldProblem]) ++ Seq(
+        // DirectMissingMethodProblem by flags being changed from ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC to ACC_PUBLIC in mixin forwarders:
+        // * from java.util.Comparator:
+        Seq("reversed", "thenComparing", "thenComparingInt", "thenComparingLong", "thenComparingInt", "thenComparingDouble").flatMap { method =>
+          Seq(
+            ProblemFilters.exclude[DirectMissingMethodProblem](s"scala.Enumeration#ValueOrdering.$method"),
+            ProblemFilters.exclude[DirectMissingMethodProblem](s"scala.concurrent.duration.Deadline#DeadlineIsOrdered.$method"),
+            ProblemFilters.exclude[DirectMissingMethodProblem](s"scala.concurrent.duration.Duration#DurationIsOrdered.$method"),
+            ProblemFilters.exclude[DirectMissingMethodProblem](s"scala.concurrent.duration.FiniteDuration#FiniteDurationIsOrdered.$method"),
+            ProblemFilters.exclude[DirectMissingMethodProblem](s"scala.math.Numeric#*.$method"),
+            ProblemFilters.exclude[DirectMissingMethodProblem](s"scala.math.Ordering#*.$method"),
+          )
+        },
+        // * from java.util.Spliterator:
+        Seq("getExactSizeIfKnown", "hasCharacteristics", "getComparator").flatMap { method =>
+          Seq(
+            ProblemFilters.exclude[DirectMissingMethodProblem](s"scala.collection.DoubleStepper#DoubleStepperSpliterator.$method"),
+            ProblemFilters.exclude[DirectMissingMethodProblem](s"scala.collection.IntStepper#IntStepperSpliterator.$method"),
+            ProblemFilters.exclude[DirectMissingMethodProblem](s"scala.collection.LongStepper#LongStepperSpliterator.$method"),
+          )
+        },
+        // * from java.util.Enumeration:
+        Seq(
+          ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.convert.JavaCollectionWrappers#IteratorWrapper.asIterator"),
+        ),
+        // * from java.lang.CharSequence:
+        Seq(
+          ProblemFilters.exclude[DirectMissingMethodProblem]("scala.Predef#ArrayCharSequence.isEmpty"),
+          ProblemFilters.exclude[DirectMissingMethodProblem]("scala.Predef#SeqCharSequence.isEmpty"),
+          ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.ArrayCharSequence.isEmpty")
+        ),
+        // FinalMethodProblem - seems to be a bug in MiMa, as neither new or old version do not have final modifier.
+        // What did change is that sizeHint, and requireBounds had `final` added, but that does not get reported.
+        Seq(
+          ProblemFilters.exclude[FinalMethodProblem]("scala.**.sizeHint$default$2"),
+          ProblemFilters.exclude[FinalMethodProblem]("scala.collection.mutable.ArrayDeque.requireBounds$default$2")
+        )
+      ).flatten
+
     }
   )
 }

project/Scala2LibraryBootstrappedMiMaFilters.scala

@@ -0,0 +1 @@
+public class A_2 extends C<String> { }

tests/pos/11484/A_2.java

@@ -0,0 +1,6 @@
+class B[A]
+sealed trait T[A] {
+  def overloaded(that: List[T[A]]): T[A] = that.head
+  def overloaded(that: List[B[A]]): B[A] = that.head
+}
+abstract class C[A] extends T[A]