Index: clang-tools-extra/clangd/FindTarget.cpp
===================================================================
--- clang-tools-extra/clangd/FindTarget.cpp
+++ clang-tools-extra/clangd/FindTarget.cpp
@@ -181,6 +181,9 @@
       for (const UsingShadowDecl *S : UD->shadows())
         add(S->getUnderlyingDecl(), Flags);
       Flags |= Rel::Alias; // continue with the alias.
+    } else if (const UsingEnumDecl *UED = dyn_cast<UsingEnumDecl>(D)) {
+      add(UED->getEnumDecl(), Flags);
+      Flags |= Rel::Alias; // continue with the alias.
     } else if (const auto *NAD = dyn_cast<NamespaceAliasDecl>(D)) {
       add(NAD->getUnderlyingDecl(), Flags | Rel::Underlying);
       Flags |= Rel::Alias; // continue with the alias
Index: clang/include/clang/AST/ASTContext.h
===================================================================
--- clang/include/clang/AST/ASTContext.h
+++ clang/include/clang/AST/ASTContext.h
@@ -517,6 +517,12 @@
   /// B<int> to the UnresolvedUsingDecl in B<T>.
   llvm::DenseMap<NamedDecl *, NamedDecl *> InstantiatedFromUsingDecl;
 
+  /// Like InstantiatedFromUsingDecl, but for using-enum declarations. Maps
+  /// from the instantiated using-enum to the templated decl from whence it
+  /// came.
+  llvm::DenseMap<NamedDecl *, NamedDecl *> InstantiatedFromUsingEnumDecl;
+
+  /// Simlarly maps instantiated UsingShadowDecls to their origin.
   llvm::DenseMap<UsingShadowDecl*, UsingShadowDecl*>
     InstantiatedFromUsingShadowDecl;
 
@@ -885,15 +891,15 @@
   MemberSpecializationInfo *getInstantiatedFromStaticDataMember(
                                                            const VarDecl *Var);
 
-  TemplateOrSpecializationInfo
-  getTemplateOrSpecializationInfo(const VarDecl *Var);
-
   /// Note that the static data member \p Inst is an instantiation of
   /// the static data member template \p Tmpl of a class template.
   void setInstantiatedFromStaticDataMember(VarDecl *Inst, VarDecl *Tmpl,
                                            TemplateSpecializationKind TSK,
                         SourceLocation PointOfInstantiation = SourceLocation());
 
+  TemplateOrSpecializationInfo
+  getTemplateOrSpecializationInfo(const VarDecl *Var);
+
   void setTemplateOrSpecializationInfo(VarDecl *Inst,
                                        TemplateOrSpecializationInfo TSI);
 
@@ -906,9 +912,18 @@
   /// of the using decl \p Pattern of a class template.
   void setInstantiatedFromUsingDecl(NamedDecl *Inst, NamedDecl *Pattern);
 
+  /// If the given using-enum decl \p Inst is an instantiation of a
+  /// (possibly unresolved) using decl from a template instantiation,
+  /// return it.
+  NamedDecl *getInstantiatedFromUsingEnumDecl(NamedDecl *Inst);
+
+  /// Remember that the using enum decl \p Inst is an instantiation
+  /// of the using enum decl \p Pattern of a class template.
+  void setInstantiatedFromUsingEnumDecl(NamedDecl *Inst, NamedDecl *Pattern);
+
+  UsingShadowDecl *getInstantiatedFromUsingShadowDecl(UsingShadowDecl *Inst);
   void setInstantiatedFromUsingShadowDecl(UsingShadowDecl *Inst,
                                           UsingShadowDecl *Pattern);
-  UsingShadowDecl *getInstantiatedFromUsingShadowDecl(UsingShadowDecl *Inst);
 
   FieldDecl *getInstantiatedFromUnnamedFieldDecl(FieldDecl *Field);
 
Index: clang/include/clang/AST/DeclCXX.h
===================================================================
--- clang/include/clang/AST/DeclCXX.h
+++ clang/include/clang/AST/DeclCXX.h
@@ -3356,7 +3356,7 @@
   void removeShadowDecl(UsingShadowDecl *S);
 
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
-  static bool classofKind(Kind K) { return K == Using; }
+  static bool classofKind(Kind K) { return K == Using || K == UsingEnum; }
 };
 
 /// Represents a shadow constructor declaration introduced into a
@@ -3552,6 +3552,65 @@
   static bool classofKind(Kind K) { return K == Using; }
 };
 
+/// Represents a C++ using-enum-declaration.
+///
+/// For example:
+/// \code
+///    using enum SomeEnumTag ;
+/// \endcode
+
+class UsingEnumDecl : public BaseUsingDecl, public Mergeable<UsingEnumDecl> {
+  /// The source location of the 'using' keyword itself.
+  SourceLocation UsingLocation;
+
+  /// Location of the 'enum' keyword.
+  SourceLocation EnumLocation;
+
+  /// The enum
+  EnumDecl *Enum;
+
+  UsingEnumDecl(DeclContext *DC, SourceLocation UL, SourceLocation EL,
+                SourceLocation NL, EnumDecl *ED)
+      : BaseUsingDecl(UsingEnum, DC, NL, ED->getDeclName()), UsingLocation(UL),
+        EnumLocation(EL), Enum(ED) {}
+
+  void anchor() override;
+
+public:
+  friend class ASTDeclReader;
+  friend class ASTDeclWriter;
+
+  /// The source location of the 'using' keyword.
+  SourceLocation getUsingLoc() const { return UsingLocation; }
+  void setUsingLoc(SourceLocation L) { UsingLocation = L; }
+
+  /// The source location of the 'enum' keyword.
+  SourceLocation getEnumLoc() const { return EnumLocation; }
+  void setEnumLoc(SourceLocation L) { EnumLocation = L; }
+
+public:
+  EnumDecl *getEnumDecl() const { return Enum; }
+
+  static UsingEnumDecl *Create(ASTContext &C, DeclContext *DC,
+                               SourceLocation UsingL, SourceLocation EnumL,
+                               SourceLocation NameL, EnumDecl *ED);
+
+  static UsingEnumDecl *CreateDeserialized(ASTContext &C, unsigned ID);
+
+  SourceRange getSourceRange() const override LLVM_READONLY;
+
+  /// Retrieves the canonical declaration of this declaration.
+  UsingEnumDecl *getCanonicalDecl() override {
+    return cast<UsingEnumDecl>(getFirstDecl());
+  }
+  const UsingEnumDecl *getCanonicalDecl() const {
+    return cast<UsingEnumDecl>(getFirstDecl());
+  }
+
+  static bool classof(const Decl *D) { return classofKind(D->getKind()); }
+  static bool classofKind(Kind K) { return K == UsingEnum; }
+};
+
 /// Represents a pack of using declarations that a single
 /// using-declarator pack-expanded into.
 ///
Index: clang/include/clang/AST/JSONNodeDumper.h
===================================================================
--- clang/include/clang/AST/JSONNodeDumper.h
+++ clang/include/clang/AST/JSONNodeDumper.h
@@ -234,6 +234,7 @@
   void VisitUsingDirectiveDecl(const UsingDirectiveDecl *UDD);
   void VisitNamespaceAliasDecl(const NamespaceAliasDecl *NAD);
   void VisitUsingDecl(const UsingDecl *UD);
+  void VisitUsingEnumDecl(const UsingEnumDecl *UED);
   void VisitUsingShadowDecl(const UsingShadowDecl *USD);
   void VisitVarDecl(const VarDecl *VD);
   void VisitFieldDecl(const FieldDecl *FD);
Index: clang/include/clang/AST/RecursiveASTVisitor.h
===================================================================
--- clang/include/clang/AST/RecursiveASTVisitor.h
+++ clang/include/clang/AST/RecursiveASTVisitor.h
@@ -1593,6 +1593,8 @@
   TRY_TO(TraverseDeclarationNameInfo(D->getNameInfo()));
 })
 
+DEF_TRAVERSE_DECL(UsingEnumDecl, {})
+
 DEF_TRAVERSE_DECL(UsingPackDecl, {})
 
 DEF_TRAVERSE_DECL(UsingDirectiveDecl, {
Index: clang/include/clang/AST/TextNodeDumper.h
===================================================================
--- clang/include/clang/AST/TextNodeDumper.h
+++ clang/include/clang/AST/TextNodeDumper.h
@@ -350,6 +350,7 @@
   void VisitUsingDecl(const UsingDecl *D);
   void VisitUnresolvedUsingTypenameDecl(const UnresolvedUsingTypenameDecl *D);
   void VisitUnresolvedUsingValueDecl(const UnresolvedUsingValueDecl *D);
+  void VisitUsingEnumDecl(const UsingEnumDecl *D);
   void VisitUsingShadowDecl(const UsingShadowDecl *D);
   void VisitConstructorUsingShadowDecl(const ConstructorUsingShadowDecl *D);
   void VisitLinkageSpecDecl(const LinkageSpecDecl *D);
Index: clang/include/clang/Basic/DeclNodes.td
===================================================================
--- clang/include/clang/Basic/DeclNodes.td
+++ clang/include/clang/Basic/DeclNodes.td
@@ -73,6 +73,7 @@
     def Concept : DeclNode<Template>;
   def BaseUsing : DeclNode<Named, "", 1>;
     def Using : DeclNode<BaseUsing>;
+    def UsingEnum : DeclNode<BaseUsing>;
   def UsingPack : DeclNode<Named>;
   def UsingShadow : DeclNode<Named>;
     def ConstructorUsingShadow : DeclNode<UsingShadow>;
Index: clang/include/clang/Basic/DiagnosticParseKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticParseKinds.td
+++ clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -580,6 +580,12 @@
 def err_expected_catch : Error<"expected catch">;
 def err_using_namespace_in_class : Error<
   "'using namespace' is not allowed in classes">;
+def warn_cxx17_compat_using_enum_declaration : Warning<
+  "using enum declaration is incompatible with C++ standards before C++20">,
+  InGroup<CXXPre20Compat>, DefaultIgnore;
+def ext_using_enum_declaration : ExtWarn<
+  "using enum declaration is a C++20 extension">,
+  InGroup<CXX20>;
 def err_constructor_bad_name : Error<
   "missing return type for function %0; did you mean the constructor name %1?">;
 def err_destructor_tilde_identifier : Error<
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -549,8 +549,6 @@
 def ext_using_decl_class_member_enumerator : ExtWarn<
   "member using declaration naming a non-member enumerator is "
   "a C++20 extension">, InGroup<CXX20>;
-def err_using_enum_lacks_definition : Error<
-  "enum named by using-enum declaration lacks a definition">;
 def err_using_enum_is_dependent : Error<
   "using-enum cannot name a dependent type">;
 def err_ambiguous_inherited_constructor : Error<
@@ -587,6 +585,9 @@
 def note_using_decl : Note<"%select{|previous }0using declaration">;
 def err_using_decl_redeclaration_expansion : Error<
   "using declaration pack expansion at block scope produces multiple values">;
+def err_using_enum_decl_redeclaration : Error<
+  "redeclaration of using-enum declaration">;
+def note_using_enum_decl : Note<"%select{|previous }0using-enum declaration">;
 
 def warn_access_decl_deprecated : Warning<
   "access declarations are deprecated; use using declarations instead">,
@@ -1749,6 +1750,8 @@
 // C++ name lookup
 def err_incomplete_nested_name_spec : Error<
   "incomplete type %0 named in nested name specifier">;
+def err_incomplete_enum : Error<
+  "enumeration %0 is incomplete">;
 def err_dependent_nested_name_spec : Error<
   "nested name specifier for a declaration cannot depend on a template "
   "parameter">;
Index: clang/include/clang/Index/IndexSymbol.h
===================================================================
--- clang/include/clang/Index/IndexSymbol.h
+++ clang/include/clang/Index/IndexSymbol.h
@@ -75,6 +75,7 @@
   AccessorSetter,
   UsingTypename,
   UsingValue,
+  UsingEnum,
 };
 
 typedef uint16_t SymbolPropertySet;
Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -5671,6 +5671,7 @@
                                SourceLocation IdentLoc,
                                IdentifierInfo *Ident);
 
+  void FilterUsingLookup(Scope *S, LookupResult &lookup);
   void HideUsingShadowDecl(Scope *S, UsingShadowDecl *Shadow);
   bool CheckUsingShadowDecl(BaseUsingDecl *BUD, NamedDecl *Target,
                             const LookupResult &PreviousDecls,
@@ -5696,6 +5697,10 @@
       bool HasTypenameKeyword, SourceLocation TypenameLoc, CXXScopeSpec &SS,
       DeclarationNameInfo NameInfo, SourceLocation EllipsisLoc,
       const ParsedAttributesView &AttrList, bool IsInstantiation);
+  NamedDecl *BuildUsingEnumDeclaration(Scope *S, AccessSpecifier AS,
+                                       SourceLocation UsingLoc,
+                                       SourceLocation EnumLoc,
+                                       SourceLocation NameLoc, EnumDecl *ED);
   NamedDecl *BuildUsingPackDecl(NamedDecl *InstantiatedFrom,
                                 ArrayRef<NamedDecl *> Expansions);
 
@@ -5713,6 +5718,9 @@
                               SourceLocation TypenameLoc, CXXScopeSpec &SS,
                               UnqualifiedId &Name, SourceLocation EllipsisLoc,
                               const ParsedAttributesView &AttrList);
+  Decl *ActOnUsingEnumDeclaration(Scope *CurScope, AccessSpecifier AS,
+                                  SourceLocation UsingLoc,
+                                  SourceLocation EnumLoc, const DeclSpec &);
   Decl *ActOnAliasDeclaration(Scope *CurScope, AccessSpecifier AS,
                               MultiTemplateParamsArg TemplateParams,
                               SourceLocation UsingLoc, UnqualifiedId &Name,
Index: clang/include/clang/Sema/Template.h
===================================================================
--- clang/include/clang/Sema/Template.h
+++ clang/include/clang/Sema/Template.h
@@ -537,6 +537,8 @@
     Decl *VisitDecl(Decl *D);
     Decl *VisitVarDecl(VarDecl *D, bool InstantiatingVarTemplate,
                        ArrayRef<BindingDecl *> *Bindings = nullptr);
+    Decl *VisitBaseUsingDecls(BaseUsingDecl *D, BaseUsingDecl *Inst,
+                              LookupResult *Lookup);
 
     // Enable late instantiation of attributes.  Late instantiated attributes
     // will be stored in LA.
Index: clang/include/clang/Serialization/ASTBitCodes.h
===================================================================
--- clang/include/clang/Serialization/ASTBitCodes.h
+++ clang/include/clang/Serialization/ASTBitCodes.h
@@ -1339,6 +1339,9 @@
   /// A UsingDecl record.
   DECL_USING,
 
+  /// A UsingEnumDecl record.
+  DECL_USING_ENUM,
+
   /// A UsingPackDecl record.
   DECL_USING_PACK,
 
Index: clang/lib/AST/ASTContext.cpp
===================================================================
--- clang/lib/AST/ASTContext.cpp
+++ clang/lib/AST/ASTContext.cpp
@@ -1569,6 +1569,24 @@
   InstantiatedFromUsingDecl[Inst] = Pattern;
 }
 
+NamedDecl *ASTContext::getInstantiatedFromUsingEnumDecl(NamedDecl *UUD) {
+  auto Pos = InstantiatedFromUsingEnumDecl.find(UUD);
+  if (Pos == InstantiatedFromUsingEnumDecl.end())
+    return nullptr;
+
+  return Pos->second;
+}
+
+void ASTContext::setInstantiatedFromUsingEnumDecl(NamedDecl *Inst,
+                                                  NamedDecl *Pattern) {
+  assert(isa<UsingEnumDecl>(Pattern) &&
+         "pattern decl is not a using-enum decl");
+  assert(isa<UsingEnumDecl>(Inst) &&
+         "instantiation did not produce a using-enum decl");
+  assert(!InstantiatedFromUsingEnumDecl[Inst] && "pattern already exists");
+  InstantiatedFromUsingEnumDecl[Inst] = Pattern;
+}
+
 UsingShadowDecl *
 ASTContext::getInstantiatedFromUsingShadowDecl(UsingShadowDecl *Inst) {
   llvm::DenseMap<UsingShadowDecl*, UsingShadowDecl*>::const_iterator Pos
Index: clang/lib/AST/ASTImporter.cpp
===================================================================
--- clang/lib/AST/ASTImporter.cpp
+++ clang/lib/AST/ASTImporter.cpp
@@ -512,6 +512,8 @@
     ExpectedDecl VisitUsingDecl(UsingDecl *D);
     ExpectedDecl VisitUsingShadowDecl(UsingShadowDecl *D);
     ExpectedDecl VisitUsingDirectiveDecl(UsingDirectiveDecl *D);
+    ExpectedDecl ImportUsingShadowDecls(BaseUsingDecl *D, BaseUsingDecl *ToSI);
+    ExpectedDecl VisitUsingEnumDecl(UsingEnumDecl *D);
     ExpectedDecl VisitUnresolvedUsingValueDecl(UnresolvedUsingValueDecl *D);
     ExpectedDecl VisitUnresolvedUsingTypenameDecl(UnresolvedUsingTypenameDecl *D);
     ExpectedDecl VisitBuiltinTemplateDecl(BuiltinTemplateDecl *D);
@@ -4560,6 +4562,19 @@
   return ToLinkageSpec;
 }
 
+ExpectedDecl ASTNodeImporter::ImportUsingShadowDecls(BaseUsingDecl *D,
+                                                     BaseUsingDecl *ToSI) {
+  for (UsingShadowDecl *FromShadow : D->shadows()) {
+    if (Expected<UsingShadowDecl *> ToShadowOrErr = import(FromShadow))
+      ToSI->addShadowDecl(*ToShadowOrErr);
+    else
+      // FIXME: We return error here but the definition is already created
+      // and available with lookups. How to fix this?..
+      return ToShadowOrErr.takeError();
+  }
+  return ToSI;
+}
+
 ExpectedDecl ASTNodeImporter::VisitUsingDecl(UsingDecl *D) {
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
@@ -4599,15 +4614,44 @@
       return ToPatternOrErr.takeError();
   }
 
-  for (UsingShadowDecl *FromShadow : D->shadows()) {
-    if (Expected<UsingShadowDecl *> ToShadowOrErr = import(FromShadow))
-      ToUsing->addShadowDecl(*ToShadowOrErr);
+  return ImportUsingShadowDecls(D, ToUsing);
+}
+
+ExpectedDecl ASTNodeImporter::VisitUsingEnumDecl(UsingEnumDecl *D) {
+  DeclContext *DC, *LexicalDC;
+  DeclarationName Name;
+  SourceLocation Loc;
+  NamedDecl *ToD = nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
+  if (ToD)
+    return ToD;
+
+  Error Err = Error::success();
+  auto ToUsingLoc = importChecked(Err, D->getUsingLoc());
+  auto ToEnumLoc = importChecked(Err, D->getEnumLoc());
+  auto ToEnumDecl = importChecked(Err, D->getEnumDecl());
+  if (Err)
+    return std::move(Err);
+
+  UsingEnumDecl *ToUsingEnum;
+  if (GetImportedOrCreateDecl(ToUsingEnum, D, Importer.getToContext(), DC,
+                              ToUsingLoc, ToEnumLoc, Loc, ToEnumDecl))
+    return ToUsingEnum;
+
+  ToUsingEnum->setLexicalDeclContext(LexicalDC);
+  LexicalDC->addDeclInternal(ToUsingEnum);
+
+  if (NamedDecl *FromPattern =
+          Importer.getFromContext().getInstantiatedFromUsingEnumDecl(D)) {
+    if (Expected<NamedDecl *> ToPatternOrErr = import(FromPattern))
+      Importer.getToContext().setInstantiatedFromUsingEnumDecl(ToUsingEnum,
+                                                               *ToPatternOrErr);
     else
-      // FIXME: We return error here but the definition is already created
-      // and available with lookups. How to fix this?..
-      return ToShadowOrErr.takeError();
+      return ToPatternOrErr.takeError();
   }
-  return ToUsing;
+
+  return ImportUsingShadowDecls(D, ToUsingEnum);
 }
 
 ExpectedDecl ASTNodeImporter::VisitUsingShadowDecl(UsingShadowDecl *D) {
Index: clang/lib/AST/Decl.cpp
===================================================================
--- clang/lib/AST/Decl.cpp
+++ clang/lib/AST/Decl.cpp
@@ -1370,6 +1370,7 @@
     case Decl::NamespaceAlias:
     case Decl::ParmVar:
     case Decl::Using:
+    case Decl::UsingEnum:
     case Decl::UsingShadow:
     case Decl::UsingDirective:
       return LinkageInfo::none();
Index: clang/lib/AST/DeclBase.cpp
===================================================================
--- clang/lib/AST/DeclBase.cpp
+++ clang/lib/AST/DeclBase.cpp
@@ -784,6 +784,7 @@
 
     case Using:
     case UsingPack:
+    case UsingEnum:
       return IDNS_Using;
 
     case ObjCProtocol:
Index: clang/lib/AST/DeclCXX.cpp
===================================================================
--- clang/lib/AST/DeclCXX.cpp
+++ clang/lib/AST/DeclCXX.cpp
@@ -3080,6 +3080,23 @@
   return SourceRange(Begin, getNameInfo().getEndLoc());
 }
 
+void UsingEnumDecl::anchor() {}
+
+UsingEnumDecl *UsingEnumDecl::Create(ASTContext &C, DeclContext *DC,
+                                     SourceLocation UL, SourceLocation EL,
+                                     SourceLocation NL, EnumDecl *Enum) {
+  return new (C, DC) UsingEnumDecl(DC, UL, EL, NL, Enum);
+}
+
+UsingEnumDecl *UsingEnumDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
+  return new (C, ID) UsingEnumDecl(nullptr, SourceLocation(), SourceLocation(),
+                                   SourceLocation(), nullptr);
+}
+
+SourceRange UsingEnumDecl::getSourceRange() const {
+  return SourceRange(EnumLocation, getLocation());
+}
+
 void UsingPackDecl::anchor() {}
 
 UsingPackDecl *UsingPackDecl::Create(ASTContext &C, DeclContext *DC,
Index: clang/lib/AST/DeclPrinter.cpp
===================================================================
--- clang/lib/AST/DeclPrinter.cpp
+++ clang/lib/AST/DeclPrinter.cpp
@@ -98,6 +98,7 @@
     void VisitUnresolvedUsingTypenameDecl(UnresolvedUsingTypenameDecl *D);
     void VisitUnresolvedUsingValueDecl(UnresolvedUsingValueDecl *D);
     void VisitUsingDecl(UsingDecl *D);
+    void VisitUsingEnumDecl(UsingEnumDecl *D);
     void VisitUsingShadowDecl(UsingShadowDecl *D);
     void VisitOMPThreadPrivateDecl(OMPThreadPrivateDecl *D);
     void VisitOMPAllocateDecl(OMPAllocateDecl *D);
@@ -1609,6 +1610,10 @@
   Out << *D;
 }
 
+void DeclPrinter::VisitUsingEnumDecl(UsingEnumDecl *D) {
+  Out << "using enum " << D->getEnumDecl();
+}
+
 void
 DeclPrinter::VisitUnresolvedUsingTypenameDecl(UnresolvedUsingTypenameDecl *D) {
   Out << "using typename ";
Index: clang/lib/AST/JSONNodeDumper.cpp
===================================================================
--- clang/lib/AST/JSONNodeDumper.cpp
+++ clang/lib/AST/JSONNodeDumper.cpp
@@ -756,6 +756,10 @@
   JOS.attribute("name", Name);
 }
 
+void JSONNodeDumper::VisitUsingEnumDecl(const UsingEnumDecl *UED) {
+  JOS.attribute("target", createBareDeclRef(UED->getEnumDecl()));
+}
+
 void JSONNodeDumper::VisitUsingShadowDecl(const UsingShadowDecl *USD) {
   JOS.attribute("target", createBareDeclRef(USD->getTargetDecl()));
 }
Index: clang/lib/AST/TextNodeDumper.cpp
===================================================================
--- clang/lib/AST/TextNodeDumper.cpp
+++ clang/lib/AST/TextNodeDumper.cpp
@@ -2051,6 +2051,11 @@
   OS << D->getDeclName();
 }
 
+void TextNodeDumper::VisitUsingEnumDecl(const UsingEnumDecl *D) {
+  OS << ' ';
+  dumpBareDeclRef(D->getEnumDecl());
+}
+
 void TextNodeDumper::VisitUnresolvedUsingTypenameDecl(
     const UnresolvedUsingTypenameDecl *D) {
   OS << ' ';
Index: clang/lib/CodeGen/CGDebugInfo.h
===================================================================
--- clang/lib/CodeGen/CGDebugInfo.h
+++ clang/lib/CodeGen/CGDebugInfo.h
@@ -508,6 +508,9 @@
   /// Emit C++ using declaration.
   void EmitUsingDecl(const UsingDecl &UD);
 
+  /// Emit C++ using-enum declaration.
+  void EmitUsingEnumDecl(const UsingEnumDecl &UD);
+
   /// Emit an @import declaration.
   void EmitImportDecl(const ImportDecl &ID);
 
Index: clang/lib/CodeGen/CGDebugInfo.cpp
===================================================================
--- clang/lib/CodeGen/CGDebugInfo.cpp
+++ clang/lib/CodeGen/CGDebugInfo.cpp
@@ -4995,6 +4995,17 @@
   }
 }
 
+void CGDebugInfo::EmitUsingEnumDecl(const UsingEnumDecl &UD) {
+  if (!CGM.getCodeGenOpts().hasReducedDebugInfo())
+    return;
+  assert(UD.shadow_size() &&
+         "We shouldn't be codegening an invalid UsingEnumDecl"
+         " containing no decls");
+
+  for (const auto *USD : UD.shadows())
+    EmitUsingShadowDecl(*USD);
+}
+
 void CGDebugInfo::EmitImportDecl(const ImportDecl &ID) {
   if (CGM.getCodeGenOpts().getDebuggerTuning() != llvm::DebuggerKind::LLDB)
     return;
Index: clang/lib/CodeGen/CGDecl.cpp
===================================================================
--- clang/lib/CodeGen/CGDecl.cpp
+++ clang/lib/CodeGen/CGDecl.cpp
@@ -137,6 +137,10 @@
     if (CGDebugInfo *DI = getDebugInfo())
         DI->EmitUsingDecl(cast<UsingDecl>(D));
     return;
+  case Decl::UsingEnum: // using enum X; [C++]
+    if (CGDebugInfo *DI = getDebugInfo())
+      DI->EmitUsingEnumDecl(cast<UsingEnumDecl>(D));
+    return;
   case Decl::UsingPack:
     for (auto *Using : cast<UsingPackDecl>(D).expansions())
       EmitDecl(*Using);
Index: clang/lib/CodeGen/CodeGenModule.cpp
===================================================================
--- clang/lib/CodeGen/CodeGenModule.cpp
+++ clang/lib/CodeGen/CodeGenModule.cpp
@@ -5723,6 +5723,10 @@
     if (CGDebugInfo *DI = getModuleDebugInfo())
         DI->EmitUsingDecl(cast<UsingDecl>(*D));
     break;
+  case Decl::UsingEnum: // using enum X; [C++]
+    if (CGDebugInfo *DI = getModuleDebugInfo())
+      DI->EmitUsingEnumDecl(cast<UsingEnumDecl>(*D));
+    break;
   case Decl::NamespaceAlias:
     if (CGDebugInfo *DI = getModuleDebugInfo())
         DI->EmitNamespaceAlias(cast<NamespaceAliasDecl>(*D));
Index: clang/lib/Index/IndexSymbol.cpp
===================================================================
--- clang/lib/Index/IndexSymbol.cpp
+++ clang/lib/Index/IndexSymbol.cpp
@@ -329,6 +329,11 @@
       Info.Kind = SymbolKind::Using;
       Info.Lang = SymbolLanguage::CXX;
       break;
+    case Decl::UsingEnum:
+      Info.Kind = SymbolKind::Using;
+      Info.Lang = SymbolLanguage::CXX;
+      Info.SubKind = SymbolSubKind::UsingEnum;
+      break;
     case Decl::Binding:
       Info.Kind = SymbolKind::Variable;
       Info.Lang = SymbolLanguage::CXX;
@@ -542,6 +547,8 @@
   case SymbolSubKind::AccessorSetter: return "acc-set";
   case SymbolSubKind::UsingTypename: return "using-typename";
   case SymbolSubKind::UsingValue: return "using-value";
+  case SymbolSubKind::UsingEnum:
+    return "using-enum";
   }
   llvm_unreachable("invalid symbol subkind");
 }
Index: clang/lib/Parse/ParseDeclCXX.cpp
===================================================================
--- clang/lib/Parse/ParseDeclCXX.cpp
+++ clang/lib/Parse/ParseDeclCXX.cpp
@@ -670,11 +670,46 @@
 ///     alias-declaration: C++11 [dcl.dcl]p1
 ///       'using' identifier attribute-specifier-seq[opt] = type-id ;
 ///
+///     using-enum-declaration: [C++20, dcl.enum]
+///       'using' elaborated-enum-specifier ;
+///
+///     elaborated-enum-specifier:
+///       'enum' nested-name-specifier[opt] identifier
 Parser::DeclGroupPtrTy
 Parser::ParseUsingDeclaration(DeclaratorContext Context,
                               const ParsedTemplateInfo &TemplateInfo,
                               SourceLocation UsingLoc, SourceLocation &DeclEnd,
                               AccessSpecifier AS) {
+  SourceLocation UELoc;
+  if (TryConsumeToken(tok::kw_enum, UELoc)) {
+    // C++20 using-enum
+    Diag(UELoc, getLangOpts().CPlusPlus20
+                    ? diag::warn_cxx17_compat_using_enum_declaration
+                    : diag::ext_using_enum_declaration);
+
+    DeclSpec DS(AttrFactory);
+    ParseEnumSpecifier(UELoc, DS, TemplateInfo, AS,
+                       // DSC_trailing has the semantics we desire
+                       DeclSpecContext::DSC_trailing);
+
+    if (TemplateInfo.Kind) {
+      SourceRange R = TemplateInfo.getSourceRange();
+      Diag(UsingLoc, diag::err_templated_using_directive_declaration)
+          << 1 /* declaration */ << R << FixItHint::CreateRemoval(R);
+
+      return nullptr;
+    }
+
+    Decl *UED = Actions.ActOnUsingEnumDeclaration(getCurScope(), AS, UsingLoc,
+                                                  UELoc, DS);
+    DeclEnd = Tok.getLocation();
+    if (ExpectAndConsume(tok::semi, diag::err_expected_after,
+                         "using-enum declaration"))
+      SkipUntil(tok::semi);
+
+    return Actions.ConvertDeclToDeclGroup(UED);
+  }
+
   // Check for misplaced attributes before the identifier in an
   // alias-declaration.
   ParsedAttributesWithRange MisplacedAttrs(AttrFactory);
@@ -771,8 +806,9 @@
   // Eat ';'.
   DeclEnd = Tok.getLocation();
   if (ExpectAndConsume(tok::semi, diag::err_expected_after,
-                       !Attrs.empty() ? "attributes list"
-                                      : "using declaration"))
+                       !Attrs.empty()    ? "attributes list"
+                       : UELoc.isValid() ? "using-enum declaration"
+                                         : "using declaration"))
     SkipUntil(tok::semi);
 
   return Actions.BuildDeclaratorGroup(DeclsInGroup);
Index: clang/lib/Sema/SemaCXXScopeSpec.cpp
===================================================================
--- clang/lib/Sema/SemaCXXScopeSpec.cpp
+++ clang/lib/Sema/SemaCXXScopeSpec.cpp
@@ -240,7 +240,6 @@
 ///
 bool Sema::RequireCompleteEnumDecl(EnumDecl *EnumD, SourceLocation L,
                                    CXXScopeSpec *SS) {
-  assert (SS && "missing scope");
   if (EnumD->isCompleteDefinition()) {
     // If we know about the definition but it is not visible, complain.
     NamedDecl *SuggestedDef = nullptr;
@@ -264,16 +263,22 @@
       if (InstantiateEnum(L, EnumD, Pattern,
                           getTemplateInstantiationArgs(EnumD),
                           TSK_ImplicitInstantiation)) {
-        SS->SetInvalid(SS->getRange());
+        if (SS)
+          SS->SetInvalid(SS->getRange());
         return true;
       }
       return false;
     }
   }
 
-  Diag(L, diag::err_incomplete_nested_name_spec)
-      << QualType(EnumD->getTypeForDecl(), 0) << SS->getRange();
-  SS->SetInvalid(SS->getRange());
+  if (SS) {
+    Diag(L, diag::err_incomplete_nested_name_spec)
+        << QualType(EnumD->getTypeForDecl(), 0) << SS->getRange();
+    SS->SetInvalid(SS->getRange());
+  } else {
+    Diag(L, diag::err_incomplete_enum) << QualType(EnumD->getTypeForDecl(), 0);
+    Diag(EnumD->getLocation(), diag::note_declared_at);
+  }
 
   return true;
 }
Index: clang/lib/Sema/SemaCodeComplete.cpp
===================================================================
--- clang/lib/Sema/SemaCodeComplete.cpp
+++ clang/lib/Sema/SemaCodeComplete.cpp
@@ -3915,6 +3915,9 @@
   case Decl::UnresolvedUsingTypename:
     return CXCursor_UsingDeclaration;
 
+  case Decl::UsingEnum:
+    return CXCursor_EnumDecl;
+
   case Decl::ObjCPropertyImpl:
     switch (cast<ObjCPropertyImplDecl>(D)->getPropertyImplementation()) {
     case ObjCPropertyImplDecl::Dynamic:
Index: clang/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- clang/lib/Sema/SemaDeclCXX.cpp
+++ clang/lib/Sema/SemaDeclCXX.cpp
@@ -1835,9 +1835,11 @@
     case Decl::UsingDirective:
     case Decl::UnresolvedUsingTypename:
     case Decl::UnresolvedUsingValue:
+    case Decl::UsingEnum:
       //   - static_assert-declarations
       //   - using-declarations,
       //   - using-directives,
+      //   - using-enum-declaration
       continue;
 
     case Decl::Typedef:
@@ -11599,7 +11601,41 @@
   NamedDecl *UD =
       BuildUsingDeclaration(S, AS, UsingLoc, TypenameLoc.isValid(), TypenameLoc,
                             SS, TargetNameInfo, EllipsisLoc, AttrList,
-                            /*IsInstantiation*/false);
+                            /*IsInstantiation*/ false);
+  if (UD)
+    PushOnScopeChains(UD, S, /*AddToContext*/ false);
+
+  return UD;
+}
+
+Decl *Sema::ActOnUsingEnumDeclaration(Scope *S, AccessSpecifier AS,
+                                      SourceLocation UsingLoc,
+                                      SourceLocation EnumLoc,
+                                      const DeclSpec &DS) {
+  switch (DS.getTypeSpecType()) {
+  case DeclSpec::TST_error:
+    // This will already have been diagnosed
+    return nullptr;
+
+  case DeclSpec::TST_enum:
+    break;
+
+  case DeclSpec::TST_typename:
+    Diag(DS.getTypeSpecTypeLoc(), diag::err_using_enum_is_dependent);
+    return nullptr;
+
+  default:
+    llvm_unreachable("unexpected DeclSpec type");
+  }
+
+  // As with enum-decls, we ignore attributes for now.
+
+  auto *Enum = cast<EnumDecl>(DS.getRepAsDecl());
+  if (auto *Def = Enum->getDefinition())
+    Enum = Def;
+
+  auto *UD = BuildUsingEnumDeclaration(S, AS, UsingLoc, EnumLoc,
+                                       DS.getTypeSpecTypeNameLoc(), Enum);
   if (UD)
     PushOnScopeChains(UD, S, /*AddToContext*/ false);
 
@@ -11697,7 +11733,7 @@
     // We can have UsingDecls in our Previous results because we use the same
     // LookupResult for checking whether the UsingDecl itself is a valid
     // redeclaration.
-    if (isa<UsingDecl>(D) || isa<UsingPackDecl>(D))
+    if (isa<UsingDecl>(D) || isa<UsingPackDecl>(D) || isa<UsingEnumDecl>(D))
       continue;
 
     if (auto *RD = dyn_cast<CXXRecordDecl>(D)) {
@@ -11985,6 +12021,29 @@
 };
 } // end anonymous namespace
 
+/// Remove decls we can't actually see from a lookup being used to declare
+/// shadow using decls.
+///
+/// \param S - The scope of the potential shadow decl
+/// \param Previous - The lookup of a potential shadow decl's name.
+void Sema::FilterUsingLookup(Scope *S, LookupResult &Previous) {
+  // It is really dumb that we have to do this.
+  LookupResult::Filter F = Previous.makeFilter();
+  while (F.hasNext()) {
+    NamedDecl *D = F.next();
+    if (!isDeclInScope(D, CurContext, S))
+      F.erase();
+    // If we found a local extern declaration that's not ordinarily visible,
+    // and this declaration is being added to a non-block scope, ignore it.
+    // We're only checking for scope conflicts here, not also for violations
+    // of the linkage rules.
+    else if (!CurContext->isFunctionOrMethod() && D->isLocalExternDecl() &&
+             !(D->getIdentifierNamespace() & Decl::IDNS_Ordinary))
+      F.erase();
+  }
+  F.done();
+}
+
 /// Builds a using declaration.
 ///
 /// \param IsInstantiation - Whether this call arises from an
@@ -12017,21 +12076,7 @@
   if (S) {
     LookupName(Previous, S);
 
-    // It is really dumb that we have to do this.
-    LookupResult::Filter F = Previous.makeFilter();
-    while (F.hasNext()) {
-      NamedDecl *D = F.next();
-      if (!isDeclInScope(D, CurContext, S))
-        F.erase();
-      // If we found a local extern declaration that's not ordinarily visible,
-      // and this declaration is being added to a non-block scope, ignore it.
-      // We're only checking for scope conflicts here, not also for violations
-      // of the linkage rules.
-      else if (!CurContext->isFunctionOrMethod() && D->isLocalExternDecl() &&
-               !(D->getIdentifierNamespace() & Decl::IDNS_Ordinary))
-        F.erase();
-    }
-    F.done();
+    FilterUsingLookup(S, Previous);
   } else {
     assert(IsInstantiation && "no scope in non-instantiation");
     if (CurContext->isRecord())
@@ -12243,6 +12288,62 @@
   return UD;
 }
 
+NamedDecl *Sema::BuildUsingEnumDeclaration(Scope *S, AccessSpecifier AS,
+                                           SourceLocation UsingLoc,
+                                           SourceLocation EnumLoc,
+                                           SourceLocation NameLoc,
+                                           EnumDecl *ED) {
+  bool Invalid = false;
+
+  if (CurContext->getRedeclContext()->isRecord()) {
+    /// In class scope, check if this is a duplicate, for better a diagnostic.
+    DeclarationNameInfo UsingEnumName(ED->getDeclName(), NameLoc);
+    LookupResult Previous(*this, UsingEnumName, LookupUsingDeclName,
+                          ForVisibleRedeclaration);
+
+    LookupName(Previous, S);
+
+    for (NamedDecl *D : Previous)
+      if (UsingEnumDecl *UED = dyn_cast<UsingEnumDecl>(D))
+        if (UED->getEnumDecl() == ED) {
+          Diag(UsingLoc, diag::err_using_enum_decl_redeclaration)
+              << SourceRange(EnumLoc, NameLoc);
+          Diag(D->getLocation(), diag::note_using_enum_decl) << 1;
+          Invalid = true;
+          break;
+        }
+  }
+
+  if (RequireCompleteEnumDecl(ED, NameLoc))
+    Invalid = true;
+
+  UsingEnumDecl *UD = UsingEnumDecl::Create(Context, CurContext, UsingLoc,
+                                            EnumLoc, NameLoc, ED);
+  UD->setAccess(AS);
+  CurContext->addDecl(UD);
+
+  if (Invalid) {
+    UD->setInvalidDecl();
+    return UD;
+  }
+
+  // Create the shadow decls for each enumerator
+  for (EnumConstantDecl *EC : ED->enumerators()) {
+    UsingShadowDecl *PrevDecl = nullptr;
+    DeclarationNameInfo DNI(EC->getDeclName(), EC->getLocation());
+    LookupResult Previous(*this, DNI, LookupOrdinaryName,
+                          ForVisibleRedeclaration);
+    LookupName(Previous, S);
+    FilterUsingLookup(S, Previous);
+
+    if (!CheckUsingShadowDecl(UD, EC, Previous, PrevDecl)) {
+      BuildUsingShadowDecl(S, UD, EC, PrevDecl);
+    }
+  }
+
+  return UD;
+}
+
 NamedDecl *Sema::BuildUsingPackDecl(NamedDecl *InstantiatedFrom,
                                     ArrayRef<NamedDecl *> Expansions) {
   assert(isa<UnresolvedUsingValueDecl>(InstantiatedFrom) ||
Index: clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
===================================================================
--- clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -2979,6 +2979,47 @@
   return Inst;
 }
 
+Decl *TemplateDeclInstantiator::VisitBaseUsingDecls(BaseUsingDecl *D,
+                                                    BaseUsingDecl *Inst,
+                                                    LookupResult *Lookup) {
+
+  bool isFunctionScope = Owner->isFunctionOrMethod();
+
+  for (auto *Shadow : D->shadows()) {
+    // FIXME: UsingShadowDecl doesn't preserve its immediate target, so
+    // reconstruct it in the case where it matters. Hm, can we extract it from
+    // the DeclSpec when parsing and save it in the UsingDecl itself?
+    NamedDecl *OldTarget = Shadow->getTargetDecl();
+    if (auto *CUSD = dyn_cast<ConstructorUsingShadowDecl>(Shadow))
+      if (auto *BaseShadow = CUSD->getNominatedBaseClassShadowDecl())
+        OldTarget = BaseShadow;
+
+    NamedDecl *InstTarget =
+        cast_or_null<NamedDecl>(SemaRef.FindInstantiatedDecl(
+            Shadow->getLocation(), OldTarget, TemplateArgs));
+    if (!InstTarget)
+      return nullptr;
+
+    UsingShadowDecl *PrevDecl = nullptr;
+    if (Lookup &&
+        SemaRef.CheckUsingShadowDecl(Inst, InstTarget, *Lookup, PrevDecl))
+      continue;
+
+    if (UsingShadowDecl *OldPrev = getPreviousDeclForInstantiation(Shadow))
+      PrevDecl = cast_or_null<UsingShadowDecl>(SemaRef.FindInstantiatedDecl(
+          Shadow->getLocation(), OldPrev, TemplateArgs));
+
+    UsingShadowDecl *InstShadow = SemaRef.BuildUsingShadowDecl(
+        /*Scope*/ nullptr, Inst, InstTarget, PrevDecl);
+    SemaRef.Context.setInstantiatedFromUsingShadowDecl(InstShadow, Shadow);
+
+    if (isFunctionScope)
+      SemaRef.CurrentInstantiationScope->InstantiatedLocal(Shadow, InstShadow);
+  }
+
+  return Inst;
+}
+
 Decl *TemplateDeclInstantiator::VisitUsingDecl(UsingDecl *D) {
 
   // The nested name specifier may be dependent, for example
@@ -3004,11 +3045,9 @@
       NameInfo.setName(SemaRef.Context.DeclarationNames.getCXXConstructorName(
           SemaRef.Context.getCanonicalType(SemaRef.Context.getRecordType(RD))));
 
-  // We only need to do redeclaration lookups if we're in a class
-  // scope (in fact, it's not really even possible in non-class
-  // scopes).
+  // We only need to do redeclaration lookups if we're in a class scope (in
+  // fact, it's not really even possible in non-class scopes).
   bool CheckRedeclaration = Owner->isRecord();
-
   LookupResult Prev(SemaRef, NameInfo, Sema::LookupUsingDeclName,
                     Sema::ForVisibleRedeclaration);
 
@@ -3029,13 +3068,13 @@
                                             D->hasTypename(), SS,
                                             D->getLocation(), Prev))
       NewUD->setInvalidDecl();
-
   }
 
   if (!NewUD->isInvalidDecl() &&
       SemaRef.CheckUsingDeclQualifier(D->getUsingLoc(), D->hasTypename(), SS,
                                       NameInfo, D->getLocation(), nullptr, D))
     NewUD->setInvalidDecl();
+
   SemaRef.Context.setInstantiatedFromUsingDecl(NewUD, D);
   NewUD->setAccess(D->getAccess());
   Owner->addDecl(NewUD);
@@ -3049,43 +3088,34 @@
   if (NameInfo.getName().getNameKind() == DeclarationName::CXXConstructorName)
     SemaRef.CheckInheritingConstructorUsingDecl(NewUD);
 
-  bool isFunctionScope = Owner->isFunctionOrMethod();
+  return VisitBaseUsingDecls(D, NewUD, CheckRedeclaration ? &Prev : nullptr);
+}
 
-  // Process the shadow decls.
-  for (auto *Shadow : D->shadows()) {
-    // FIXME: UsingShadowDecl doesn't preserve its immediate target, so
-    // reconstruct it in the case where it matters.
-    NamedDecl *OldTarget = Shadow->getTargetDecl();
-    if (auto *CUSD = dyn_cast<ConstructorUsingShadowDecl>(Shadow))
-      if (auto *BaseShadow = CUSD->getNominatedBaseClassShadowDecl())
-        OldTarget = BaseShadow;
+Decl *TemplateDeclInstantiator::VisitUsingEnumDecl(UsingEnumDecl *D) {
+  // Cannot be a dependent type, but still could be an instantiation
+  EnumDecl *EnumD = cast_or_null<EnumDecl>(SemaRef.FindInstantiatedDecl(
+      D->getLocation(), D->getEnumDecl(), TemplateArgs));
 
-    NamedDecl *InstTarget =
-        cast_or_null<NamedDecl>(SemaRef.FindInstantiatedDecl(
-            Shadow->getLocation(), OldTarget, TemplateArgs));
-    if (!InstTarget)
-      return nullptr;
+  if (SemaRef.RequireCompleteEnumDecl(EnumD, EnumD->getLocation()))
+    return nullptr;
 
-    UsingShadowDecl *PrevDecl = nullptr;
-    if (CheckRedeclaration) {
-      if (SemaRef.CheckUsingShadowDecl(NewUD, InstTarget, Prev, PrevDecl))
-        continue;
-    } else if (UsingShadowDecl *OldPrev =
-                   getPreviousDeclForInstantiation(Shadow)) {
-      PrevDecl = cast_or_null<UsingShadowDecl>(SemaRef.FindInstantiatedDecl(
-          Shadow->getLocation(), OldPrev, TemplateArgs));
-    }
+  UsingEnumDecl *NewUD =
+      UsingEnumDecl::Create(SemaRef.Context, Owner, D->getUsingLoc(),
+                            D->getEnumLoc(), D->getLocation(), EnumD);
 
-    UsingShadowDecl *InstShadow =
-        SemaRef.BuildUsingShadowDecl(/*Scope*/nullptr, NewUD, InstTarget,
-                                     PrevDecl);
-    SemaRef.Context.setInstantiatedFromUsingShadowDecl(InstShadow, Shadow);
+  SemaRef.Context.setInstantiatedFromUsingEnumDecl(NewUD, D);
+  NewUD->setAccess(D->getAccess());
+  Owner->addDecl(NewUD);
 
-    if (isFunctionScope)
-      SemaRef.CurrentInstantiationScope->InstantiatedLocal(Shadow, InstShadow);
-  }
+  // Don't process the shadow decls for an invalid decl.
+  if (NewUD->isInvalidDecl())
+    return NewUD;
+
+  // We don't have to recheck for duplication of the UsingEnumDecl itself, as it
+  // cannot be dependent, and will therefore have been checked during template
+  // definition.
 
-  return NewUD;
+  return VisitBaseUsingDecls(D, NewUD, nullptr);
 }
 
 Decl *TemplateDeclInstantiator::VisitUsingShadowDecl(UsingShadowDecl *D) {
Index: clang/lib/Serialization/ASTCommon.cpp
===================================================================
--- clang/lib/Serialization/ASTCommon.cpp
+++ clang/lib/Serialization/ASTCommon.cpp
@@ -394,6 +394,7 @@
   case Decl::NonTypeTemplateParm:
   case Decl::TemplateTemplateParm:
   case Decl::Using:
+  case Decl::UsingEnum:
   case Decl::UsingPack:
   case Decl::ObjCMethod:
   case Decl::ObjCCategory:
Index: clang/lib/Serialization/ASTReaderDecl.cpp
===================================================================
--- clang/lib/Serialization/ASTReaderDecl.cpp
+++ clang/lib/Serialization/ASTReaderDecl.cpp
@@ -389,6 +389,7 @@
     void VisitTemplateTemplateParmDecl(TemplateTemplateParmDecl *D);
     void VisitTypeAliasTemplateDecl(TypeAliasTemplateDecl *D);
     void VisitUsingDecl(UsingDecl *D);
+    void VisitUsingEnumDecl(UsingEnumDecl *D);
     void VisitUsingPackDecl(UsingPackDecl *D);
     void VisitUsingShadowDecl(UsingShadowDecl *D);
     void VisitConstructorUsingShadowDecl(ConstructorUsingShadowDecl *D);
@@ -1651,6 +1652,17 @@
   mergeMergeable(D);
 }
 
+void ASTDeclReader::VisitUsingEnumDecl(UsingEnumDecl *D) {
+  VisitNamedDecl(D);
+  D->setUsingLoc(readSourceLocation());
+  D->setEnumLoc(readSourceLocation());
+  D->Enum = readDeclAs<EnumDecl>();
+  D->FirstUsingShadow.setPointer(readDeclAs<UsingShadowDecl>());
+  if (auto *Pattern = readDeclAs<NamedDecl>())
+    Reader.getContext().setInstantiatedFromUsingEnumDecl(D, Pattern);
+  mergeMergeable(D);
+}
+
 void ASTDeclReader::VisitUsingPackDecl(UsingPackDecl *D) {
   VisitNamedDecl(D);
   D->InstantiatedFrom = readDeclAs<NamedDecl>();
@@ -3837,6 +3849,9 @@
   case DECL_USING_SHADOW:
     D = UsingShadowDecl::CreateDeserialized(Context, ID);
     break;
+  case DECL_USING_ENUM:
+    D = UsingEnumDecl::CreateDeserialized(Context, ID);
+    break;
   case DECL_CONSTRUCTOR_USING_SHADOW:
     D = ConstructorUsingShadowDecl::CreateDeserialized(Context, ID);
     break;
Index: clang/lib/Serialization/ASTWriterDecl.cpp
===================================================================
--- clang/lib/Serialization/ASTWriterDecl.cpp
+++ clang/lib/Serialization/ASTWriterDecl.cpp
@@ -113,6 +113,7 @@
     void VisitTemplateTemplateParmDecl(TemplateTemplateParmDecl *D);
     void VisitTypeAliasTemplateDecl(TypeAliasTemplateDecl *D);
     void VisitUsingDecl(UsingDecl *D);
+    void VisitUsingEnumDecl(UsingEnumDecl *D);
     void VisitUsingPackDecl(UsingPackDecl *D);
     void VisitUsingShadowDecl(UsingShadowDecl *D);
     void VisitConstructorUsingShadowDecl(ConstructorUsingShadowDecl *D);
@@ -1276,6 +1277,16 @@
   Code = serialization::DECL_USING;
 }
 
+void ASTDeclWriter::VisitUsingEnumDecl(UsingEnumDecl *D) {
+  VisitNamedDecl(D);
+  Record.AddSourceLocation(D->getUsingLoc());
+  Record.AddSourceLocation(D->getEnumLoc());
+  Record.AddDeclRef(D->getEnumDecl());
+  Record.AddDeclRef(D->FirstUsingShadow.getPointer());
+  Record.AddDeclRef(Context.getInstantiatedFromUsingEnumDecl(D));
+  Code = serialization::DECL_USING_ENUM;
+}
+
 void ASTDeclWriter::VisitUsingPackDecl(UsingPackDecl *D) {
   Record.push_back(D->NumExpansions);
   VisitNamedDecl(D);
Index: clang/test/SemaCXX/cxx20-using-enum.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaCXX/cxx20-using-enum.cpp
@@ -0,0 +1,233 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++17 -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++20 -verify %s
+
+// p1099 'using enum ELABORATED-ENUM-SPECIFIER ;'
+
+namespace One {
+namespace Bob {
+enum A { a, // expected-note{{declared here}}
+         b,
+         c };
+class C; // expected-note{{previous use}}
+enum class D : int;
+enum class D { d,
+               e,
+               f };
+enum class D : int;
+} // namespace Bob
+
+using enum Bob::A;
+#if __cplusplus < 202002
+// expected-warning@-2{{is a C++20 extension}}
+#endif
+using enum Bob::B; // expected-error{{no enum named 'B'}}
+#if __cplusplus < 202002
+// expected-warning@-2{{is a C++20 extension}}
+#endif
+using enum Bob::C; // expected-error{{tag type that does not match}}
+#if __cplusplus < 202002
+// expected-warning@-2{{is a C++20 extension}}
+#endif
+auto v = a;
+
+A g; // expected-error{{unknown type name 'A'}}
+
+int A;
+
+using enum Bob::D;
+#if __cplusplus < 202002
+// expected-warning@-2{{is a C++20 extension}}
+#endif
+} // namespace One
+
+namespace Two {
+namespace Kevin {
+enum class B { d,
+               e,
+               f };
+}
+
+using enum Kevin::B;
+#if __cplusplus < 202002
+// expected-warning@-2{{is a C++20 extension}}
+#endif
+auto w = e;
+
+} // namespace Two
+
+#if __cplusplus >= 202002
+// Now only check c++20 onwards
+
+namespace Three {
+namespace Stuart {
+enum class C : int; // expected-note{{declared here}}
+}
+
+using enum Stuart::C; // expected-error{{is incomplete}}
+} // namespace Three
+
+namespace Four {
+class Dave {
+public:
+  enum D { a,
+           b,
+           c };
+
+private:
+  enum class E { d, // expected-note{{declared private here}}
+                 e,
+                 f };
+};
+
+using enum Dave::D;
+using enum Dave::E; // expected-error{{is a private member}}
+
+} // namespace Four
+
+namespace Five {
+enum class A { b,
+               c };
+class Dave {
+public:
+  using enum A;
+  A f = b;
+};
+
+} // namespace Five
+
+namespace Six {
+template <typename T> class TPL;
+template <> class TPL<int> {
+public:
+  enum A { a };
+};
+
+template <typename T> class USR {
+  using enum TPL<T>::B; // expected-error{{cannot name a dependent type}}
+  using enum TPL<int>::A;
+};
+} // namespace Six
+
+// Now instantiate things
+namespace Seven {
+namespace Stuart {
+enum class A { a,
+               b,
+               c };
+}
+
+static_assert(!int(Stuart::A::a));
+constexpr int Bar() {
+  using enum Stuart::A;
+  return int(b);
+}
+static_assert(Bar() == 1);
+
+template <int I> constexpr int Foo() {
+  using enum Stuart::A;
+  return int(b) + I;
+}
+
+static_assert(Foo<10>() == 11);
+
+template <int I> struct C {
+  using enum Stuart::A;
+  static constexpr int V = int(c) + I;
+
+  enum class D { d,
+                 e,
+                 f };
+  using enum D;
+
+  static constexpr int W = int(f) + I;
+};
+
+static_assert(C<2>::V == 4);
+static_assert(C<20>::W == 22);
+
+} // namespace Seven
+
+namespace Eight {
+enum class Bob : int {};
+using enum Bob;
+} // namespace Eight
+
+namespace Nine {
+template <int I> struct C {
+  enum class D { i = I };
+  enum class E : int; // expected-note{{declared here}}
+};
+
+using enum C<2>::D;
+
+constexpr auto d = i;
+static_assert(unsigned(d) == 2);
+
+using enum C<2>::E; // expected-error{{instantiation of undefined member}}
+} // namespace Nine
+
+namespace Ten {
+enum class Bob { a };
+
+void Foo() {
+  extern void a();
+}
+
+// We don't see the hidden extern a fn!
+using enum Bob;
+
+auto v = a;
+} // namespace Ten
+
+namespace Eleven {
+enum class Bob { a }; // expected-note{{conflicting declaration}}
+
+struct Base {
+  enum { a }; // expected-note{{target of using}}
+};
+
+template <typename B>
+class TPLa : B {
+  using enum Bob;
+  using B::a; // expected-error{{target of using declaration}}
+};
+
+TPLa<Base> a; // expected-note{{in instantiation}}
+
+} // namespace Eleven
+
+namespace Twelve {
+enum class Bob { a }; // expected-note{{target of using}}
+
+struct Base {
+  enum { a };
+};
+
+template <typename B>
+class TPLb : B {
+  using B::a;     // expected-note{{conflicting declaration}}
+  using enum Bob; // expected-error{{target of using declaration}}
+};
+
+TPLb<Base> b;
+
+} // namespace Twelve
+
+namespace Thirteen {
+enum class Bob { a };
+class Foo {
+  using enum Bob; // expected-note{{previous using-enum}}
+  using enum Bob; // expected-error{{redeclaration of using-enum}}
+};
+
+template <typename B>
+class TPLa {
+  using enum Bob; // expected-note{{previous using-enum}}
+  using enum Bob; // expected-error{{redeclaration of using-enum}}
+};
+
+TPLa<int> a;
+
+} // namespace Thirteen
+
+#endif
Index: clang/tools/libclang/CIndex.cpp
===================================================================
--- clang/tools/libclang/CIndex.cpp
+++ clang/tools/libclang/CIndex.cpp
@@ -6540,6 +6540,7 @@
   }
 
   case Decl::Using:
+  case Decl::UsingEnum:
     return MakeCursorOverloadedDeclRef(cast<BaseUsingDecl>(D), D->getLocation(),
                                        TU);