diff --git a/compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala b/compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala
index 03d5ac13665b..75391ae5c23f 100644
--- a/compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala
+++ b/compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala
@@ -615,14 +615,11 @@ trait BCodeHelpers extends BCodeIdiomatic with BytecodeWriters {
      *  The members are sorted by name and signature to guarantee a stable ordering.
      */
     private def sortedMembersBasedOnFlags(tp: Type, required: Flag, excluded: FlagSet): List[Symbol] = {
-      // The output of `memberNames` is a Set, sort it to guarantee a stable ordering.
-      val names = tp.memberNames(takeAllFilter).toSeq.sorted
-      val buffer = mutable.ListBuffer[Symbol]()
-      names.foreach { name =>
-        buffer ++= tp.memberBasedOnFlags(name, required, excluded)
+      val names = tp.memberNames(takeAllFilter).toList
+      names.flatMap { name =>
+        tp.memberBasedOnFlags(name, required, excluded)
           .alternatives.sortBy(_.signature)(Signature.lexicographicOrdering).map(_.symbol)
       }
-      buffer.toList
     }
 
     /*
diff --git a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
index 7f47f1696cd6..52c99122fafa 100644
--- a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -22,6 +22,7 @@ import java.util.WeakHashMap
 import scala.util.control.NonFatal
 import config.Config
 import reporting._
+import collection.immutable.SortedSet
 import collection.mutable
 import transform.TypeUtils._
 
@@ -2231,7 +2232,7 @@ object SymDenotations {
       }
     }
 
-    def memberNames(keepOnly: NameFilter)(implicit onBehalf: MemberNames, ctx: Context): Set[Name] =
+    def memberNames(keepOnly: NameFilter)(implicit onBehalf: MemberNames, ctx: Context): SortedSet[Name] =
       if (this.is(PackageClass) || !Config.cacheMemberNames)
         computeMemberNames(keepOnly) // don't cache package member names; they might change
       else {
@@ -2239,8 +2240,8 @@ object SymDenotations {
         memberNamesCache(keepOnly, this)
       }
 
-    def computeMemberNames(keepOnly: NameFilter)(implicit onBehalf: MemberNames, ctx: Context): Set[Name] = {
-      var names = Set[Name]()
+    def computeMemberNames(keepOnly: NameFilter)(implicit onBehalf: MemberNames, ctx: Context): SortedSet[Name] = {
+      var names = SortedSet[Name]()
       def maybeAdd(name: Name) = if (keepOnly(thisType, name)) names += name
       try {
         for (p <- parentSyms if p.isClass)
@@ -2466,8 +2467,8 @@ object SymDenotations {
     end computeMembersNamed
 
     /** The union of the member names of the package and the package object */
-    override def memberNames(keepOnly: NameFilter)(implicit onBehalf: MemberNames, ctx: Context): Set[Name] = {
-      def recur(pobjs: List[ClassDenotation], acc: Set[Name]): Set[Name] = pobjs match {
+    override def memberNames(keepOnly: NameFilter)(implicit onBehalf: MemberNames, ctx: Context): SortedSet[Name] = {
+      def recur(pobjs: List[ClassDenotation], acc: SortedSet[Name]): SortedSet[Name] = pobjs match {
         case pcls :: pobjs1 =>
           recur(pobjs1, acc.union(pcls.memberNames(keepOnly)))
         case nil =>
@@ -2752,7 +2753,7 @@ object SymDenotations {
   /** A cache for sets of member names, indexed by a NameFilter */
   trait MemberNames extends InheritedCache {
     def apply(keepOnly: NameFilter, clsd: ClassDenotation)
-             (implicit onBehalf: MemberNames, ctx: Context): Set[Name]
+             (implicit onBehalf: MemberNames, ctx: Context): SortedSet[Name]
   }
 
   object MemberNames {
@@ -2813,7 +2814,7 @@ object SymDenotations {
   }
 
   private class MemberNamesImpl(createdAt: Period) extends InheritedCacheImpl(createdAt) with MemberNames {
-    private var cache: SimpleIdentityMap[NameFilter, Set[Name]] = SimpleIdentityMap.empty
+    private var cache: SimpleIdentityMap[NameFilter, SortedSet[Name]] = SimpleIdentityMap.empty
 
     final def isValid(using Context): Boolean =
       cache != null && isValidAt(ctx.phase)
diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index b384cbdcb084..fe0f309029ed 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -29,6 +29,7 @@ import printing.Texts._
 import printing.Printer
 import Hashable._
 import Uniques._
+import collection.immutable.SortedSet
 import collection.mutable
 import config.Config
 import config.Feature
@@ -902,7 +903,7 @@ object Types {
      *  @note: OK to use a Set[Name] here because Name hashcodes are replayable,
      *         hence the Set will always give the same names in the same order.
      */
-    final def memberNames(keepOnly: NameFilter, pre: Type = this)(using Context): Set[Name] = this match {
+    final def memberNames(keepOnly: NameFilter, pre: Type = this)(using Context): SortedSet[Name] = this match {
       case tp: ClassInfo =>
         val names = tp.cls.classDenot.memberNames(keepOnly)
         if keepOnly.isStable then names else names.filter(keepOnly(pre, _))
@@ -916,7 +917,7 @@ object Types {
       case tp: OrType =>
         tp.tp1.memberNames(keepOnly, pre) & tp.tp2.memberNames(keepOnly, pre)
       case _ =>
-        Set()
+        SortedSet()
     }
 
     def memberDenots(keepOnly: NameFilter, f: (Name, mutable.Buffer[SingleDenotation]) => Unit)(using Context): Seq[SingleDenotation] = {
diff --git a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
index 4c20f5821ca4..f3866f0b0f41 100644
--- a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
+++ b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
@@ -51,9 +51,9 @@ object RefChecks {
     }
     {
       val defaultGetterNames = defaultGetterClass.asClass.memberNames(defaultMethodFilter)
-      val defaultMethodNames = defaultGetterNames map { _ replace {
+      val defaultMethodNames = defaultGetterNames.map { _ replace {
         case DefaultGetterName(methName, _) => methName
-      }}
+      }}(Names.NameOrdering)
 
       for (name <- defaultMethodNames) {
         val methods = clazz.info.member(name).alternatives.map(_.symbol)