diff --git a/compiler/src/dotty/tools/backend/sjs/JSExportsGen.scala b/compiler/src/dotty/tools/backend/sjs/JSExportsGen.scala
index c571a08980a1..eba914f1aa98 100644
--- a/compiler/src/dotty/tools/backend/sjs/JSExportsGen.scala
+++ b/compiler/src/dotty/tools/backend/sjs/JSExportsGen.scala
@@ -1083,14 +1083,14 @@ final class JSExportsGen(jsCodeGen: JSCodeGen)(using Context) {
       js.VarRef(js.LocalIdent(restParamName))(jstpe.AnyType)
     }
 
-    def genAllArgsRefsForForwarder()(implicit pos: Position): List[js.Tree] = {
+    def genAllArgsRefsForForwarder()(implicit pos: Position): List[js.TreeOrJSSpread] = {
       val fixedArgRefs = fixedParamNames.toList.map { paramName =>
         js.VarRef(js.LocalIdent(paramName))(jstpe.AnyType)
       }
 
       if (needsRestParam) {
         val restArgRef = js.VarRef(js.LocalIdent(restParamName))(jstpe.AnyType)
-        fixedArgRefs :+ restArgRef
+        fixedArgRefs :+ js.JSSpread(restArgRef)
       } else {
         fixedArgRefs
       }
diff --git a/project/Build.scala b/project/Build.scala
index 20cde25461cf..f443a7c20d0a 100644
--- a/project/Build.scala
+++ b/project/Build.scala
@@ -104,8 +104,8 @@ object Build {
    *  scala-library.
    */
   def stdlibVersion(implicit mode: Mode): String = mode match {
-    case NonBootstrapped => "2.13.5"
-    case Bootstrapped => "2.13.5"
+    case NonBootstrapped => "2.13.6"
+    case Bootstrapped => "2.13.6"
   }
 
   val dottyOrganization = "org.scala-lang"
@@ -1173,12 +1173,10 @@ object Build {
             -- "ObjectTest.scala" // compile errors caused by #9588
             -- "StackTraceTest.scala" // would require `npm install source-map-support`
             -- "UnionTypeTest.scala" // requires the Scala 2 macro defined in Typechecking*.scala
-            -- "PromiseMock.scala" // TODO: Enable once we use a Scala.js with https://github.com/scala-js/scala-js/pull/4451 in
-                                   // and remove copy in tests/sjs-junit
             )).get
 
           ++ (dir / "js/src/test/require-2.12" ** (("*.scala": FileFilter)
-            -- "JSOptionalTest212.scala" // TODO: Enable once we use a Scala.js with https://github.com/scala-js/scala-js/pull/4451 in
+            -- "JSOptionalTest212FunParamInference.scala" // TODO: #11694
             )).get
           ++ (dir / "js/src/test/require-sam" ** "*.scala").get
           ++ (dir / "js/src/test/scala-new-collections" ** "*.scala").get
@@ -1387,6 +1385,7 @@ object Build {
       scriptedLaunchOpts ++= Seq(
         "-Dplugin.version=" + version.value,
         "-Dplugin.scalaVersion=" + dottyVersion,
+        "-Dplugin.scala2Version=" + stdlibVersion(Bootstrapped),
         "-Dplugin.scalaJSVersion=" + scalaJSVersion,
         "-Dsbt.boot.directory=" + ((ThisBuild / baseDirectory).value / ".sbt-scripted").getAbsolutePath // Workaround sbt/sbt#3469
       ),
diff --git a/project/build.properties b/project/build.properties
index e67343ae796c..19479ba46ff2 100644
--- a/project/build.properties
+++ b/project/build.properties
@@ -1 +1 @@
-sbt.version=1.5.0
+sbt.version=1.5.2
diff --git a/project/plugins.sbt b/project/plugins.sbt
index e0071ea63676..b1a292ec9624 100644
--- a/project/plugins.sbt
+++ b/project/plugins.sbt
@@ -2,7 +2,7 @@
 //
 // e.g. addSbtPlugin("com.github.mpeltonen" % "sbt-idea" % "1.1.0")
 
-addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.5.0")
+addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.5.1")
 
 addSbtPlugin("org.xerial.sbt" % "sbt-sonatype" % "3.6")
 
diff --git a/sbt-test/scala2-compat/erasure-scalajs/build.sbt b/sbt-test/scala2-compat/erasure-scalajs/build.sbt
index 6f30dbadc25b..342c32fc4cdc 100644
--- a/sbt-test/scala2-compat/erasure-scalajs/build.sbt
+++ b/sbt-test/scala2-compat/erasure-scalajs/build.sbt
@@ -1,7 +1,7 @@
 lazy val scala2Lib = project.in(file("scala2Lib"))
   .enablePlugins(ScalaJSPlugin)
   .settings(
-    scalaVersion := "2.13.5"
+    scalaVersion := sys.props("plugin.scala2Version")
   )
 
 lazy val dottyApp = project.in(file("dottyApp"))
diff --git a/sbt-test/scala2-compat/erasure/build.sbt b/sbt-test/scala2-compat/erasure/build.sbt
index 794d78b308c0..694101e79388 100644
--- a/sbt-test/scala2-compat/erasure/build.sbt
+++ b/sbt-test/scala2-compat/erasure/build.sbt
@@ -1,6 +1,6 @@
 lazy val scala2Lib = project.in(file("scala2Lib"))
   .settings(
-    scalaVersion := "2.13.5"
+    scalaVersion := sys.props("plugin.scala2Version")
   )
 
 lazy val dottyApp = project.in(file("dottyApp"))
diff --git a/sbt-test/scala2-compat/i11173/build.sbt b/sbt-test/scala2-compat/i11173/build.sbt
index 4cc8010e1d38..433a5e8baddf 100644
--- a/sbt-test/scala2-compat/i11173/build.sbt
+++ b/sbt-test/scala2-compat/i11173/build.sbt
@@ -1,5 +1,5 @@
 val scala3Version = sys.props("plugin.scalaVersion")
-val scala2Version = "2.13.5"
+val scala2Version = sys.props("plugin.scala2Version")
 
 lazy val lib = project.in(file("lib"))
   .settings(
diff --git a/sbt-test/scala2-compat/i12109/build.sbt b/sbt-test/scala2-compat/i12109/build.sbt
index 4cc8010e1d38..433a5e8baddf 100644
--- a/sbt-test/scala2-compat/i12109/build.sbt
+++ b/sbt-test/scala2-compat/i12109/build.sbt
@@ -1,5 +1,5 @@
 val scala3Version = sys.props("plugin.scalaVersion")
-val scala2Version = "2.13.5"
+val scala2Version = sys.props("plugin.scala2Version")
 
 lazy val lib = project.in(file("lib"))
   .settings(
diff --git a/sbt-test/scala2-compat/i8001/build.sbt b/sbt-test/scala2-compat/i8001/build.sbt
index 27c7d9ea9b3c..1dc85ed7cac0 100644
--- a/sbt-test/scala2-compat/i8001/build.sbt
+++ b/sbt-test/scala2-compat/i8001/build.sbt
@@ -1,5 +1,5 @@
 val scala3Version = sys.props("plugin.scalaVersion")
-val scala2Version = "2.13.5"
+val scala2Version = sys.props("plugin.scala2Version")
 
 lazy val lib = (project in file ("lib"))
   .settings(scalaVersion := scala2Version)
diff --git a/sbt-test/scala2-compat/i8847/build.sbt b/sbt-test/scala2-compat/i8847/build.sbt
index 6a579f2be966..1e7c318b5880 100644
--- a/sbt-test/scala2-compat/i8847/build.sbt
+++ b/sbt-test/scala2-compat/i8847/build.sbt
@@ -1,5 +1,5 @@
 val scala3Version = sys.props("plugin.scalaVersion")
-val scala2Version = "2.13.5"
+val scala2Version = sys.props("plugin.scala2Version")
 
 lazy val `i8847-lib` = (project in file ("lib"))
   .settings(scalaVersion := scala2Version)
diff --git a/sbt-test/scala2-compat/i9916a/build.sbt b/sbt-test/scala2-compat/i9916a/build.sbt
index 1f5c07900161..5da7a83d315f 100644
--- a/sbt-test/scala2-compat/i9916a/build.sbt
+++ b/sbt-test/scala2-compat/i9916a/build.sbt
@@ -1,5 +1,5 @@
 val scala3Version = sys.props("plugin.scalaVersion")
-val scala2Version = "2.13.5"
+val scala2Version = sys.props("plugin.scala2Version")
 
 lazy val `i9916a-lib` = (project in file ("lib"))
   .settings(scalaVersion := scala2Version)
diff --git a/sbt-test/scala2-compat/i9916b/build.sbt b/sbt-test/scala2-compat/i9916b/build.sbt
index f597a8610607..14a979bb173c 100644
--- a/sbt-test/scala2-compat/i9916b/build.sbt
+++ b/sbt-test/scala2-compat/i9916b/build.sbt
@@ -1,5 +1,5 @@
 val scala3Version = sys.props("plugin.scalaVersion")
-val scala2Version = "2.13.5"
+val scala2Version = sys.props("plugin.scala2Version")
 
 lazy val `i9916b-lib` = (project in file ("lib"))
   .settings(scalaVersion := scala2Version)
diff --git a/sbt-test/scala2-compat/longnames/build.sbt b/sbt-test/scala2-compat/longnames/build.sbt
index 61d8f34cd5f6..df60250f06f1 100644
--- a/sbt-test/scala2-compat/longnames/build.sbt
+++ b/sbt-test/scala2-compat/longnames/build.sbt
@@ -1,5 +1,5 @@
 val scala3Version = sys.props("plugin.scalaVersion")
-val scala2Version = "2.13.5"
+val scala2Version = sys.props("plugin.scala2Version")
 
 lazy val lib = (project in file ("lib"))
   .settings(scalaVersion := scala2Version)
diff --git a/sbt-test/scala2-compat/structural/build.sbt b/sbt-test/scala2-compat/structural/build.sbt
index 61d8f34cd5f6..df60250f06f1 100644
--- a/sbt-test/scala2-compat/structural/build.sbt
+++ b/sbt-test/scala2-compat/structural/build.sbt
@@ -1,5 +1,5 @@
 val scala3Version = sys.props("plugin.scalaVersion")
-val scala2Version = "2.13.5"
+val scala2Version = sys.props("plugin.scala2Version")
 
 lazy val lib = (project in file ("lib"))
   .settings(scalaVersion := scala2Version)
diff --git a/tests/sjs-junit/test/org/scalajs/testsuite/jsinterop/PromiseMock.scala b/tests/sjs-junit/test/org/scalajs/testsuite/jsinterop/PromiseMock.scala
deleted file mode 100644
index 6764cdc7ac83..000000000000
--- a/tests/sjs-junit/test/org/scalajs/testsuite/jsinterop/PromiseMock.scala
+++ /dev/null
@@ -1,260 +0,0 @@
-/*
- * Scala.js (https://www.scala-js.org/)
- *
- * Copyright EPFL.
- *
- * Licensed under Apache License 2.0
- * (https://www.apache.org/licenses/LICENSE-2.0).
- *
- * See the NOTICE file distributed with this work for
- * additional information regarding copyright ownership.
- */
-
-package org.scalajs.testsuite.jsinterop
-
-import scala.scalajs.js
-import scala.scalajs.js.annotation._
-import scala.scalajs.js.|
-
-import js.Thenable
-
-object PromiseMock {
-
-  @noinline
-  def withMockedPromise[A](body: (() => Unit) => A): A = {
-    val global = org.scalajs.testsuite.utils.JSUtils.globalObject
-
-    val oldPromise =
-      if (global.hasOwnProperty("Promise").asInstanceOf[Boolean]) Some(global.Promise)
-      else None
-
-    global.Promise = js.constructorOf[MockPromise[_]]
-    try {
-      body(MockPromise.processQueue _)
-    } finally {
-      oldPromise.fold {
-        js.special.delete(global, "Promise")
-      } { old =>
-        global.Promise = old
-      }
-    }
-  }
-
-  @noinline
-  def withMockedPromiseIfExists[A](body: (Option[() => Unit]) => A): A = {
-    val global = org.scalajs.testsuite.utils.JSUtils.globalObject
-
-    val oldPromise = global.Promise
-
-    if (js.isUndefined(oldPromise)) {
-      body(None)
-    } else {
-      global.Promise = js.constructorOf[MockPromise[_]]
-      try {
-        body(Some(MockPromise.processQueue _))
-      } finally {
-        global.Promise = oldPromise
-      }
-    }
-  }
-
-  private object MockPromise {
-    private val queue = js.Array[js.Function0[Any]]()
-
-    @JSExportStatic
-    def resolve[A](value: A | js.Thenable[A]): MockPromise[A] = {
-      new MockPromise[A]({
-        (resolve: js.Function1[A | js.Thenable[A], _],
-            reject: js.Function1[Any, _]) =>
-          resolve(value)
-      })
-    }
-
-    @JSExportStatic
-    def reject(reason: Any): MockPromise[Nothing] = {
-      new MockPromise[Nothing]({
-        (resolve: js.Function1[Nothing | js.Thenable[Nothing], _],
-            reject: js.Function1[Any, _]) =>
-          reject(reason)
-      })
-    }
-
-    def enqueue(f: js.Function0[Any]): Unit =
-      queue.push(f)
-
-    def processQueue(): Unit = {
-      while (queue.nonEmpty)
-        queue.shift()()
-    }
-
-    private sealed abstract class State[+A]
-
-    private case object Pending extends State[Nothing]
-    private case class Fulfilled[+A](value: A) extends State[A]
-    private case class Rejected(reason: Any) extends State[Nothing]
-
-    private def isNotAnObject(x: Any): Boolean = x match {
-      case null | () | _:Double | _:Boolean | _:String => true
-      case _                                           => false
-    }
-
-    private def isCallable(x: Any): Boolean =
-      js.typeOf(x.asInstanceOf[js.Any]) == "function"
-
-    private def throwAny(e: Any): Nothing = {
-      throw (e match {
-        case th: Throwable => th
-        case _             => js.JavaScriptException(e)
-      })
-    }
-
-    private def tryCatchAny[A](tryBody: => A)(catchBody: Any => A): A = {
-      try {
-        tryBody
-      } catch {
-        case th: Throwable =>
-          catchBody(th match {
-            case js.JavaScriptException(e) => e
-            case _                         => th
-          })
-      }
-    }
-  }
-
-  private class MockPromise[+A](
-      executor: js.Function2[js.Function1[A | Thenable[A], _], js.Function1[scala.Any, _], _])
-      extends js.Object with js.Thenable[A] {
-
-    import MockPromise._
-
-    private[this] var state: State[A] = Pending
-
-    private[this] var fulfillReactions = js.Array[js.Function1[A, Any]]()
-    private[this] var rejectReactions = js.Array[js.Function1[Any, Any]]()
-
-    init(executor)
-
-    // 25.4.3.1 Promise(executor)
-    private[this] def init(
-        executor: js.Function2[js.Function1[A | Thenable[A], _], js.Function1[scala.Any, _], _]) = {
-      tryCatchAny[Unit] {
-        executor(resolve _, reject _)
-      } { e =>
-        reject(e)
-      }
-    }
-
-    private[this] def fulfill(value: A): Unit = {
-      assert(state == Pending)
-      state = Fulfilled(value)
-      clearAndTriggerReactions(fulfillReactions, value)
-    }
-
-    private[this] def clearAndTriggerReactions[A](
-        reactions: js.Array[js.Function1[A, Any]],
-        argument: A): Unit = {
-
-      assert(state != Pending)
-
-      fulfillReactions = null
-      rejectReactions = null
-
-      for (reaction <- reactions)
-        enqueue(() => reaction(argument))
-    }
-
-    // 25.4.1.3.2 Promise Resolve Functions
-    private[this] def resolve(resolution: A | Thenable[A]): Unit = {
-      if (state == Pending) {
-        if (resolution.asInstanceOf[AnyRef] eq this) {
-          reject(new js.TypeError("Self resolution"))
-        } else if (isNotAnObject(resolution)) {
-          fulfill(resolution.asInstanceOf[A])
-        } else {
-          tryCatchAny {
-            val thenAction = resolution.asInstanceOf[js.Dynamic].`then`
-            if (!isCallable(thenAction)) {
-              fulfill(resolution.asInstanceOf[A])
-            } else {
-              val thenable = resolution.asInstanceOf[Thenable[A]]
-              val thenActionFun = thenAction.asInstanceOf[js.Function]
-              enqueue(() => promiseResolveThenableJob(thenable, thenActionFun))
-            }
-          } { e =>
-            reject(e)
-          }
-        }
-      }
-    }
-
-    // 25.4.2.2 PromiseResolveThenableJob
-    private[this] def promiseResolveThenableJob(thenable: Thenable[A],
-        thenAction: js.Function): Unit = {
-      thenAction.call(thenable, resolve _, reject _)
-    }
-
-    // 25.4.1.3.1 Promise Reject Functions
-    private[this] def reject(reason: Any): Unit = {
-      if (state == Pending) {
-        state = Rejected(reason)
-        clearAndTriggerReactions(rejectReactions, reason)
-      }
-    }
-
-    // 25.4.5.3 Promise.prototype.then
-    def `then`[B](
-        onFulfilled: js.Function1[A, B | Thenable[B]],
-        onRejected: js.UndefOr[js.Function1[scala.Any, B | Thenable[B]]]): MockPromise[B] = {
-
-      new MockPromise[B](
-        { (innerResolve: js.Function1[B | Thenable[B], _],
-            innerReject: js.Function1[scala.Any, _]) =>
-
-          def doFulfilled(value: A): Unit = {
-            tryCatchAny[Unit] {
-              innerResolve(onFulfilled(value))
-            } { e =>
-              innerReject(e)
-            }
-          }
-
-          def doRejected(reason: Any): Unit = {
-            tryCatchAny[Unit] {
-              onRejected.fold[Unit] {
-                innerReject(reason)
-              } { onRejectedFun =>
-                innerResolve(onRejectedFun(reason))
-              }
-            } { e =>
-              innerReject(e)
-            }
-          }
-
-          state match {
-            case Pending =>
-              fulfillReactions += doFulfilled _
-              rejectReactions += doRejected _
-
-            case Fulfilled(value) =>
-              enqueue(() => doFulfilled(value))
-
-            case Rejected(reason) =>
-              enqueue(() => doRejected(reason))
-          }
-        }
-      )
-    }
-
-    def `then`[B >: A](
-        onFulfilled: Unit,
-        onRejected: js.UndefOr[js.Function1[scala.Any, B | Thenable[B]]]): MockPromise[B] = {
-      `then`((x: A) => (x: B | Thenable[B]), onRejected)
-    }
-
-    // 25.4.5.1 Promise.prototype.catch
-    def `catch`[B >: A](
-        onRejected: js.UndefOr[js.Function1[scala.Any, B | Thenable[B]]]): MockPromise[B] = {
-      `then`((), onRejected)
-    }
-  }
-}