diff --git a/compiler/src/dotty/tools/dotc/core/Definitions.scala b/compiler/src/dotty/tools/dotc/core/Definitions.scala
index ca92364c5b9b..cef6a8c25714 100644
--- a/compiler/src/dotty/tools/dotc/core/Definitions.scala
+++ b/compiler/src/dotty/tools/dotc/core/Definitions.scala
@@ -1369,7 +1369,9 @@ class Definitions {
       denot.sourceModule.info = denot.typeRef // we run into a cyclic reference when patching if this line is omitted
       patch2(denot, patchCls)
 
-    if denot.name == tpnme.Predef.moduleClassName && denot.symbol == ScalaPredefModuleClass then
+    if ctx.settings.Yscala2Stdlib.value then
+      ()
+    else if denot.name == tpnme.Predef.moduleClassName && denot.symbol == ScalaPredefModuleClass then
       patchWith(ScalaPredefModuleClassPatch)
     else if denot.name == tpnme.language.moduleClassName && denot.symbol == LanguageModuleClass then
       patchWith(LanguageModuleClassPatch)
@@ -1724,8 +1726,9 @@ class Definitions {
     isFunctionType(tp) || isRefinedFunctionType(tp)
 
   private def withSpecMethods(cls: ClassSymbol, bases: List[Name], paramTypes: Set[TypeRef]) =
-    for base <- bases; tp <- paramTypes do
-      cls.enter(newSymbol(cls, base.specializedName(List(tp)), Method, ExprType(tp)))
+    if !ctx.settings.Yscala2Stdlib.value then
+      for base <- bases; tp <- paramTypes do
+        cls.enter(newSymbol(cls, base.specializedName(List(tp)), Method, ExprType(tp)))
     cls
 
   @tu lazy val Tuple1: ClassSymbol = withSpecMethods(requiredClass("scala.Tuple1"), List(nme._1), Tuple1SpecializedParamTypes)
@@ -1766,6 +1769,7 @@ class Definitions {
       case List(x, y) => Tuple2SpecializedParamClasses().contains(x.classSymbol) && Tuple2SpecializedParamClasses().contains(y.classSymbol)
       case _          => false
     && base.owner.denot.info.member(base.name.specializedName(args)).exists // when dotc compiles the stdlib there are no specialised classes
+    && !ctx.settings.Yscala2Stdlib.value // We do not add the specilized TupleN methods/classes when compiling the stdlib
 
   def isSpecializableFunction(cls: ClassSymbol, paramTypes: List[Type], retType: Type)(using Context): Boolean =
     paramTypes.length <= 2
@@ -1787,6 +1791,7 @@ class Definitions {
       case _ =>
         false
     })
+    && !ctx.settings.Yscala2Stdlib.value // We do not add the specilized FunctionN methods/classes when compiling the stdlib
 
   @tu lazy val Function0SpecializedApplyNames: collection.Set[TermName] =
     for r <- Function0SpecializedReturnTypes
diff --git a/compiler/src/dotty/tools/dotc/transform/SpecializeApplyMethods.scala b/compiler/src/dotty/tools/dotc/transform/SpecializeApplyMethods.scala
index 6ffa05075201..aae9a01698ac 100644
--- a/compiler/src/dotty/tools/dotc/transform/SpecializeApplyMethods.scala
+++ b/compiler/src/dotty/tools/dotc/transform/SpecializeApplyMethods.scala
@@ -25,7 +25,7 @@ class SpecializeApplyMethods extends MiniPhase with InfoTransformer {
   override def description: String = SpecializeApplyMethods.description
 
   override def isEnabled(using Context): Boolean =
-    !ctx.settings.scalajs.value
+    !ctx.settings.scalajs.value && !ctx.settings.Yscala2Stdlib.value
 
   private def specApplySymbol(sym: Symbol, args: List[Type], ret: Type)(using Context): Symbol = {
     val name = nme.apply.specializedFunction(ret, args)
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 458e60ddfa38..cc49694529af 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -2714,6 +2714,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
          && !cls.isAllOf(PrivateLocal)
          && effectiveOwner.is(Trait)
          && !effectiveOwner.derivesFrom(defn.ObjectClass)
+         && !ctx.settings.Yscala2Stdlib.value // FIXME?: class PermutationsItr cannot be defined in universal trait SeqOps
       then
         report.error(em"$cls cannot be defined in universal $effectiveOwner", cdef.srcPos)
 
diff --git a/project/Build.scala b/project/Build.scala
index 4356d541eb8d..a68ba05edbe3 100644
--- a/project/Build.scala
+++ b/project/Build.scala
@@ -940,6 +940,7 @@ object Build {
       },
       Compile / doc / scalacOptions += "-Ydocument-synthetic-types",
       scalacOptions += "-Yscala2-stdlib",
+      scalacOptions += "-Ycheck:all",
       scalacOptions -= "-Xfatal-warnings",
       ivyConfigurations += SourceDeps.hide,
       transitiveClassifiers := Seq("sources"),