Fix deterministically adding additional interfaces

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

@@ -113,11 +113,12 @@ 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.filter(t => !directlyInheritedTraitsSet(t) && t.is(Trait))
 //      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 +23,11 @@ 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)
+    if (!old.contains(calls))
+      superCallsMap.update(sym, old :+ calls)
   }
 
   private val entryPoints = new mutable.HashSet[String]()

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 {