diff --git a/clang/include/clang/AST/ASTContext.h b/clang/include/clang/AST/ASTContext.h
--- a/clang/include/clang/AST/ASTContext.h
+++ b/clang/include/clang/AST/ASTContext.h
@@ -408,6 +408,9 @@
   /// The identifier '__type_pack_element'.
   mutable IdentifierInfo *TypePackElementName = nullptr;
 
+  /// The identifier '__decay'
+  mutable IdentifierInfo *DecayName = nullptr;
+
   QualType ObjCConstantStringType;
   mutable RecordDecl *CFConstantStringTagDecl = nullptr;
   mutable TypedefDecl *CFConstantStringTypeDecl = nullptr;
@@ -607,6 +610,7 @@
   mutable ExternCContextDecl *ExternCContext = nullptr;
   mutable BuiltinTemplateDecl *MakeIntegerSeqDecl = nullptr;
   mutable BuiltinTemplateDecl *TypePackElementDecl = nullptr;
+  mutable BuiltinTemplateDecl *DecayDecl = nullptr;
 
   /// The associated SourceManager object.
   SourceManager &SourceMgr;
@@ -1102,6 +1106,7 @@
   ExternCContextDecl *getExternCContextDecl() const;
   BuiltinTemplateDecl *getMakeIntegerSeqDecl() const;
   BuiltinTemplateDecl *getTypePackElementDecl() const;
+  BuiltinTemplateDecl *getDecayDecl() const;
 
   // Builtin Types.
   CanQualType VoidTy;
@@ -1922,6 +1927,12 @@
     return TypePackElementName;
   }
 
+  IdentifierInfo *getDecayName() const {
+    if (!DecayName)
+      DecayName = &Idents.get("__decay");
+    return DecayName;
+  }
+
   /// Retrieve the Objective-C "instancetype" type, if already known;
   /// otherwise, returns a NULL type;
   QualType getObjCInstanceType() {
diff --git a/clang/include/clang/Basic/Builtins.h b/clang/include/clang/Basic/Builtins.h
--- a/clang/include/clang/Basic/Builtins.h
+++ b/clang/include/clang/Basic/Builtins.h
@@ -292,7 +292,10 @@
   BTK__make_integer_seq,
 
   /// This names the __type_pack_element BuiltinTemplateDecl.
-  BTK__type_pack_element
+  BTK__type_pack_element,
+
+  /// This names the __decay BuiltinTemplateDecl.
+  BTK__decay
 };
 
 } // end namespace clang
diff --git a/clang/include/clang/Serialization/ASTBitCodes.h b/clang/include/clang/Serialization/ASTBitCodes.h
--- a/clang/include/clang/Serialization/ASTBitCodes.h
+++ b/clang/include/clang/Serialization/ASTBitCodes.h
@@ -1211,6 +1211,9 @@
 
   /// The internal '__type_pack_element' template.
   PREDEF_DECL_TYPE_PACK_ELEMENT_ID = 17,
+
+  /// The internal '__decay' template.
+  PREDEF_DECL_DECAY_ID = 18,
 };
 
 /// The number of declaration IDs that are predefined.
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -1238,6 +1238,12 @@
   return TypePackElementDecl;
 }
 
+BuiltinTemplateDecl *ASTContext::getDecayDecl() const {
+  if (!DecayDecl)
+    DecayDecl = buildBuiltinTemplateDecl(BTK__decay, getDecayName());
+  return DecayDecl;
+}
+
 RecordDecl *ASTContext::buildImplicitRecord(StringRef Name,
                                             RecordDecl::TagKind TK) const {
   SourceLocation Loc;
diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -5097,6 +5097,9 @@
   case BuiltinTemplateKind::BTK__type_pack_element:
     ToD = Importer.getToContext().getTypePackElementDecl();
     break;
+  case BuiltinTemplateKind::BTK__decay:
+    ToD = Importer.getToContext().getDecayDecl();
+    break;
   }
   assert(ToD && "BuiltinTemplateDecl of unsupported kind!");
   Importer.MapImported(D, ToD);
diff --git a/clang/lib/AST/DeclTemplate.cpp b/clang/lib/AST/DeclTemplate.cpp
--- a/clang/lib/AST/DeclTemplate.cpp
+++ b/clang/lib/AST/DeclTemplate.cpp
@@ -1520,6 +1520,20 @@
                                        SourceLocation(), nullptr);
 }
 
+static TemplateParameterList *createDecayParameterList(const ASTContext &C,
+                                                       DeclContext *DC) {
+  // typename
+  auto *T = TemplateTypeParmDecl::Create(
+      C, DC, SourceLocation(), SourceLocation(), /*Depth=*/0, /*Position=*/0,
+      /*Id=*/nullptr, /*Typename=*/true, /*ParameterPack=*/false,
+      /*HasTypeConstraint=*/false);
+
+  NamedDecl *Params[] = {T};
+  return TemplateParameterList::Create(C, SourceLocation(), SourceLocation(),
+                                       llvm::makeArrayRef(Params),
+                                       SourceLocation(), nullptr);
+}
+
 static TemplateParameterList *createBuiltinTemplateParameterList(
     const ASTContext &C, DeclContext *DC, BuiltinTemplateKind BTK) {
   switch (BTK) {
@@ -1527,6 +1541,8 @@
     return createMakeIntegerSeqParameterList(C, DC);
   case BTK__type_pack_element:
     return createTypePackElementParameterList(C, DC);
+  case BTK__decay:
+    return createDecayParameterList(C, DC);
   }
 
   llvm_unreachable("unhandled BuiltinTemplateKind!");
diff --git a/clang/lib/Lex/PPMacroExpansion.cpp b/clang/lib/Lex/PPMacroExpansion.cpp
--- a/clang/lib/Lex/PPMacroExpansion.cpp
+++ b/clang/lib/Lex/PPMacroExpansion.cpp
@@ -1730,6 +1730,7 @@
               // Report builtin templates as being builtins.
               .Case("__make_integer_seq", getLangOpts().CPlusPlus)
               .Case("__type_pack_element", getLangOpts().CPlusPlus)
+              .Case("__decay", getLangOpts().CPlusPlus)
               // Likewise for some builtin preprocessor macros.
               // FIXME: This is inconsistent; we usually suggest detecting
               // builtin macros via #ifdef. Don't add more cases here.
diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -918,6 +918,9 @@
         } else if (II == getASTContext().getTypePackElementName()) {
           R.addDecl(getASTContext().getTypePackElementDecl());
           return true;
+        } else if (II == getASTContext().getDecayName()) {
+          R.addDecl(getASTContext().getDecayDecl());
+          return true;
         }
       }
 
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -3634,7 +3634,7 @@
                                        TemplateLoc, SyntheticTemplateArgs);
   }
 
-  case BTK__type_pack_element:
+  case BTK__type_pack_element: {
     // Specializations of
     //    __type_pack_element<Index, T_1, ..., T_N>
     // are treated like T_Index.
@@ -3660,6 +3660,12 @@
     int64_t N = Index.getExtValue();
     return Ts.getPackAsArray()[N].getAsType();
   }
+
+  case BTK__decay:
+    assert(Converted.size() == 1 && "__decay should be given a single type");
+    TemplateArgument Ty = Converted[0];
+    return SemaRef.BuiltinDecay(Ty.getAsType(), TemplateArgs[0].getLocation());
+  }
   llvm_unreachable("unexpected BuiltinTemplateDecl!");
 }
 
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -7433,6 +7433,9 @@
 
   case PREDEF_DECL_TYPE_PACK_ELEMENT_ID:
     return Context.getTypePackElementDecl();
+
+  case PREDEF_DECL_DECAY_ID:
+    return Context.getDecayDecl();
   }
   llvm_unreachable("PredefinedDeclIDs unknown enum value");
 }
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -4621,6 +4621,7 @@
                      PREDEF_DECL_CF_CONSTANT_STRING_TAG_ID);
   RegisterPredefDecl(Context.TypePackElementDecl,
                      PREDEF_DECL_TYPE_PACK_ELEMENT_ID);
+  RegisterPredefDecl(Context.DecayDecl, PREDEF_DECL_DECAY_ID);
 
   // Build a record containing all of the tentative definitions in this file, in
   // TentativeDefinitions order.  Generally, this record will be empty for
diff --git a/clang/test/SemaCXX/libstdcxx_transform_type_traits_hack.cpp b/clang/test/SemaCXX/libstdcxx_transform_type_traits_hack.cpp
--- a/clang/test/SemaCXX/libstdcxx_transform_type_traits_hack.cpp
+++ b/clang/test/SemaCXX/libstdcxx_transform_type_traits_hack.cpp
@@ -58,11 +58,6 @@
 template <class T>
 using J = Same<__remove_cvref<T>, __remove_cvref<T>>;
 
-template <class T>
-using __decay = int; // expected-warning{{keyword '__decay' will be made available as an identifier here}}
-template <class T>
-using K = Same<__decay<T>, __decay<T>>;
-
 template <class T>
 using __make_signed = int; // expected-warning{{keyword '__make_signed' will be made available as an identifier here}}
 template <class T>
diff --git a/clang/test/SemaCXX/type-traits.cpp b/clang/test/SemaCXX/type-traits.cpp
--- a/clang/test/SemaCXX/type-traits.cpp
+++ b/clang/test/SemaCXX/type-traits.cpp
@@ -3414,6 +3414,50 @@
   static_assert(__is_same(decay_t<int S::*const volatile &&>, int S::*), "");
 }
 
+void check_decay_template() {
+  static_assert(__is_same(__decay<void>, void), "");
+  static_assert(__is_same(__decay<const volatile void>, void), "");
+  static_assert(__is_same(__decay<int>, int), "");
+  static_assert(__is_same(__decay<const int>, int), "");
+  static_assert(__is_same(__decay<volatile int>, int), "");
+  static_assert(__is_same(__decay<const volatile int>, int), "");
+  static_assert(__is_same(__decay<int *>, int *), "");
+  static_assert(__is_same(__decay<int *const volatile>, int *), "");
+  static_assert(__is_same(__decay<int *const volatile __restrict>, int *), "");
+  static_assert(__is_same(__decay<int const *const volatile>, int const *), "");
+  static_assert(__is_same(__decay<int const *const volatile _Nonnull>, int const *), "");
+  static_assert(__is_same(__decay<int &>, int), "");
+  static_assert(__is_same(__decay<int const volatile &>, int), "");
+  static_assert(__is_same(__decay<int &&>, int), "");
+  static_assert(__is_same(__decay<int const volatile &&>, int), "");
+  static_assert(__is_same(__decay<int()>, int (*)()), "");
+  static_assert(__is_same(__decay<int (*)()>, int (*)()), "");
+  static_assert(__is_same(__decay<int (*const)()>, int (*)()), "");
+  static_assert(__is_same(__decay<int (*volatile)()>, int (*)()), "");
+  static_assert(__is_same(__decay<int (*const volatile)()>, int (*)()), "");
+  static_assert(__is_same(__decay<int (&)()>, int (*)()), "");
+  static_assert(__is_same(__decay<IntAr>, int *), "");
+  static_assert(__is_same(__decay<IntArNB>, int *), "");
+
+  static_assert(__is_same(__decay<S>, S), "");
+  static_assert(__is_same(__decay<S &>, S), "");
+  static_assert(__is_same(__decay<S &&>, S), "");
+  static_assert(__is_same(__decay<const S>, S), "");
+  static_assert(__is_same(__decay<const S &>, S), "");
+  static_assert(__is_same(__decay<const S &&>, S), "");
+  static_assert(__is_same(__decay<volatile S>, S), "");
+  static_assert(__is_same(__decay<volatile S &>, S), "");
+  static_assert(__is_same(__decay<volatile S &&>, S), "");
+  static_assert(__is_same(__decay<const volatile S>, S), "");
+  static_assert(__is_same(__decay<const volatile S &>, S), "");
+  static_assert(__is_same(__decay<const volatile S &&>, S), "");
+  static_assert(__is_same(__decay<int S::*const volatile>, int S::*), "");
+  static_assert(__is_same(__decay<int (S::*const volatile)()>, int(S::*)()), "");
+  static_assert(__is_same(__decay<int S::*const volatile &>, int S::*), "");
+  static_assert(__is_same(__decay<int (S::*const volatile &)()>, int(S::*)()), "");
+  static_assert(__is_same(__decay<int S::*const volatile &&>, int S::*), "");
+}
+
 template <class T> struct CheckAbominableFunction {};
 template <class M>
 struct CheckAbominableFunction<M S::*> {