Fix deterministically adding additional interfaces

When a class contains calls to 'super' for traits it does
not directly implement, these are added to the list of interfaces
of the generated class. Previously, because these interfaces were
determined using set logic, the ordering of that list was not
deterministic.

This makes the order deterministic assuming the order in which
these calls are registered using `registerSuperCall` in the
`CollectSuperCalls` phase is deterministic within each class.
This seems likely to me but it'd be great if someone could
confirm.

To add a test for this change, creating a class that has many
'additional' traits and checking they are generated in the right
order would make sense, but I couldn't find an obvious place for
such a test. Any recommendations?

Fixes #20496

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

@@ -113,11 +113,13 @@ class BTypesFromSymbols[I <: DottyBackendInterface](val int: I, val frontendAcce
       val directlyInheritedTraits = sym.directlyInheritedTraits
       val directlyInheritedTraitsSet = directlyInheritedTraits.toSet
       val allBaseClasses = directlyInheritedTraits.iterator.flatMap(_.asClass.baseClasses.drop(1)).toSet
-      val superCalls = superCallsMap.getOrElse(sym, Set.empty)
-      val additional = (superCalls -- directlyInheritedTraitsSet).filter(_.is(Trait))
+      val superCalls = superCallsMap.getOrElse(sym, List.empty)
+      val superCallsSet = superCalls.toSet
+      val additional = superCalls.foldLeft(List())((acc, t) =>
+        if !acc.contains(t) && !directlyInheritedTraitsSet(t) && t.is(Trait) then acc :+ t else acc)
 //      if (additional.nonEmpty)
 //        println(s"$fullName: adding supertraits $additional")
-      directlyInheritedTraits.filter(t => !allBaseClasses(t) || superCalls(t)) ++ additional
+      directlyInheritedTraits.filter(t => !allBaseClasses(t) || superCallsSet(t)) ++ additional
     }
 
     val interfaces = classSym.superInterfaces.map(classBTypeFromSymbol)

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

@@ -25,7 +25,7 @@ import StdNames.nme
 import NameKinds.{LazyBitMapName, LazyLocalName}
 import Names.Name
 
-class DottyBackendInterface(val superCallsMap: ReadOnlyMap[Symbol, Set[ClassSymbol]])(using val ctx: Context) {
+class DottyBackendInterface(val superCallsMap: ReadOnlyMap[Symbol, List[ClassSymbol]])(using val ctx: Context) {
 
   private val desugared = new java.util.IdentityHashMap[Type, tpd.Select]
 

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

@@ -23,10 +23,10 @@ class GenBCode extends Phase { self =>
 
   override def isRunnable(using Context) = super.isRunnable && !ctx.usedBestEffortTasty
 
-  private val superCallsMap = new MutableSymbolMap[Set[ClassSymbol]]
+  private val superCallsMap = new MutableSymbolMap[List[ClassSymbol]]
   def registerSuperCall(sym: Symbol, calls: ClassSymbol): Unit = {
-    val old = superCallsMap.getOrElse(sym, Set.empty)
-    superCallsMap.update(sym, old + calls)
+    val old = superCallsMap.getOrElse(sym, List.empty)
+    superCallsMap.update(sym, old :+ calls)
   }
 
   private val entryPoints = new mutable.HashSet[String]()