Only set PostTyper.changesParents to true for scala2-lib

compiler/src/dotty/tools/dotc/Run.scala

@@ -292,10 +292,13 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
       if (ctx.settings.YtestPickler.value) List("pickler")
       else ctx.settings.YstopAfter.value
 
+    val runCtx = ctx.fresh
+    runCtx.setProfiler(Profiler())
+
     val pluginPlan = ctx.base.addPluginPhases(ctx.base.phasePlan)
     val phases = ctx.base.fusePhases(pluginPlan,
       ctx.settings.Yskip.value, ctx.settings.YstopBefore.value, stopAfter, ctx.settings.Ycheck.value)
-    ctx.base.usePhases(phases)
+    ctx.base.usePhases(phases, runCtx)
 
     if ctx.settings.YnoDoubleBindings.value then
       ctx.base.checkNoDoubleBindings = true
@@ -339,9 +342,6 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
       profiler.finished()
     }
 
-    val runCtx = ctx.fresh
-    runCtx.setProfiler(Profiler())
-    unfusedPhases.foreach(_.initContext(runCtx))
     val fusedPhases = runCtx.base.allPhases
     if ctx.settings.explainCyclic.value then
       runCtx.setProperty(CyclicReference.Trace, new CyclicReference.Trace())

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

@@ -889,7 +889,7 @@ object Contexts {
     val definitions: Definitions = new Definitions
 
     // Set up some phases to get started */
-    usePhases(List(SomePhase))
+    usePhases(List(SomePhase), FreshContext(this))
 
     /** Initializes the `ContextBase` with a starting context.
      *  This initializes the `platform` and the `definitions`.

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

@@ -126,7 +126,7 @@ object Phases {
      *  The list should never contain NoPhase.
      *  if fusion is enabled, phases in same subgroup will be fused to single phase.
      */
-    final def usePhases(phasess: List[Phase], fuse: Boolean = true): Unit = {
+    final def usePhases(phasess: List[Phase], runCtx: FreshContext, fuse: Boolean = true): Unit = {
 
       val flatPhases = collection.mutable.ListBuffer[Phase]()
 
@@ -161,11 +161,21 @@ object Phases {
         phase match {
           case p: MegaPhase =>
             val miniPhases = p.miniPhases
-            miniPhases.foreach{ phase =>
+            for phase <- miniPhases do
               checkRequirements(phase)
-              phase.init(this, nextPhaseId)}
+              // Given phases a chance to initialize state based on the run context.
+              //
+              // `phase.initContext` should be called before `phase.init` as the later calls abstract methods
+              // `changesMembers` and `changeParents` which may depend on the run context.
+              //
+              // See `PostTyper.changeParents`
+              phase.initContext(runCtx)
+              phase.init(this, nextPhaseId)
+            end for
             p.init(this, miniPhases.head.id, miniPhases.last.id)
           case _ =>
+            // See comment above about the ordering of the two calls.
+            phase.initContext(runCtx)
             phase.init(this, nextPhaseId)
             checkRequirements(phase)
         }

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

@@ -76,19 +76,28 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
   override def changesMembers: Boolean = true // the phase adds super accessors and synthetic members
 
   /**
-   * Serializable and AbstractFunction are added for scala2-library companion object of case class
-   *
-   * Ideally `compilingScala2StdLib` should be used, but it is initialized too late to be effective.
+   * Serializable and AbstractFunction1 are added for companion objects of case classes in scala2-library
    */
-  override def changesParents: Boolean = true
+  override def changesParents: Boolean =
+    if !initContextCalled then
+      throw new Exception("Calling changesParents before initContext, should call initContext first")
+    compilingScala2StdLib
 
   override def transformPhase(using Context): Phase = thisPhase.next
 
   def newTransformer(using Context): Transformer =
     new PostTyperTransformer
 
+  /**
+   * Used to check that `changesParents` is called after `initContext`.
+   *
+   * This contract is easy to break and results in subtle bugs.
+   */
+  private var initContextCalled = false
+
   private var compilingScala2StdLib = false
   override def initContext(ctx: FreshContext): Unit =
+    initContextCalled = true
     compilingScala2StdLib = ctx.settings.YcompileScala2Library.value(using ctx)
 
   val superAcc: SuperAccessors = new SuperAccessors(thisPhase)
@@ -562,7 +571,7 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
         sym.addAnnotation(Annotation(defn.ExperimentalAnnot, sym.span))
 
     // It needs to run at the phase of the postTyper --- otherwise, the test of the symbols will use
-    // the transformed denotation with added `Serializable` and `AbstractFunction`.
+    // the transformed denotation with added `Serializable` and `AbstractFunction1`.
     private def scala2LibPatch(tree: TypeDef)(using Context) = atPhase(thisPhase):
       val sym = tree.symbol
       if compilingScala2StdLib && sym.is(ModuleClass) then