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 change makes the order deterministic (assuming the order in
which these calls are registered using `registerSuperCall` in the
`CollectSuperCalls` phase is deterministic within each class)

Fixes #20496

Author: raboof

Diff for: 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)

Diff for: 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]
 

Diff for: 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]()

Diff for: compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -1963,6 +1963,30 @@ class DottyBytecodeTests extends DottyBytecodeTest {
       assertSameCode(instructions, expected)
     }
   }
+
+  /**
+   * Test 'additional' imports are generated in deterministic order
+   * https://github.com/scala/scala3/issues/20496
+   */
+  @Test def deterministicAdditionalImports = {
+    val source =
+    """trait Actor:
+        |  def receive() = ()
+        |trait Timers:
+        |  def timers() = ()
+        |abstract class ShardCoordinator extends Actor with Timers
+        |class PersistentShardCoordinator extends ShardCoordinator:
+        |  def foo =
+        |    super.receive()
+        |    super.timers()""".stripMargin
+    checkBCode(source) { dir =>
+      val clsIn   = dir.lookupName("PersistentShardCoordinator.class", directory = false).input
+      val clsNode = loadClassNode(clsIn)
+
+      val expected = List("Actor", "Timers")
+      assertEquals(expected, clsNode.interfaces.asScala)
+    }
+  }
 }
 
 object invocationReceiversTestCode {