Index: clang/include/clang/AST/DeclarationName.h
===================================================================
--- clang/include/clang/AST/DeclarationName.h
+++ clang/include/clang/AST/DeclarationName.h
@@ -21,6 +21,7 @@
 #include "clang/Basic/SourceLocation.h"
 #include "llvm/ADT/DenseMapInfo.h"
 #include "llvm/ADT/FoldingSet.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/type_traits.h"
 #include <cassert>
@@ -192,6 +193,13 @@
                 "The various classes that DeclarationName::Ptr can point to"
                 " must be at least aligned to 8 bytes!");
 
+  static_assert(
+      std::is_same<std::underlying_type_t<StoredNameKind>,
+                   std::underlying_type_t<
+                       detail::DeclarationNameExtra::ExtraKind>>::value,
+      "The various enums used to compute values for NameKind should "
+      "all have the same underlying type");
+
 public:
   /// The kind of the name stored in this DeclarationName.
   /// The first 7 enumeration values are stored inline and correspond
@@ -205,15 +213,18 @@
     CXXDestructorName = StoredCXXDestructorName,
     CXXConversionFunctionName = StoredCXXConversionFunctionName,
     CXXOperatorName = StoredCXXOperatorName,
-    CXXDeductionGuideName = UncommonNameKindOffset +
-                            detail::DeclarationNameExtra::CXXDeductionGuideName,
-    CXXLiteralOperatorName =
-        UncommonNameKindOffset +
-        detail::DeclarationNameExtra::CXXLiteralOperatorName,
-    CXXUsingDirective = UncommonNameKindOffset +
-                        detail::DeclarationNameExtra::CXXUsingDirective,
-    ObjCMultiArgSelector = UncommonNameKindOffset +
-                           detail::DeclarationNameExtra::ObjCMultiArgSelector
+    CXXDeductionGuideName = llvm::addEnumValues(
+        UncommonNameKindOffset,
+        detail::DeclarationNameExtra::CXXDeductionGuideName),
+    CXXLiteralOperatorName = llvm::addEnumValues(
+        UncommonNameKindOffset,
+        detail::DeclarationNameExtra::CXXLiteralOperatorName),
+    CXXUsingDirective =
+        llvm::addEnumValues(UncommonNameKindOffset,
+                            detail::DeclarationNameExtra::CXXUsingDirective),
+    ObjCMultiArgSelector =
+        llvm::addEnumValues(UncommonNameKindOffset,
+                            detail::DeclarationNameExtra::ObjCMultiArgSelector),
   };
 
 private:
Index: llvm/include/llvm/ADT/STLExtras.h
===================================================================
--- llvm/include/llvm/ADT/STLExtras.h
+++ llvm/include/llvm/ADT/STLExtras.h
@@ -203,6 +203,17 @@
 template <size_t I, typename... Ts>
 using TypeAtIndex = std::tuple_element_t<I, std::tuple<Ts...>>;
 
+/// Helper which adds two underlying types of enumeration type.
+/// Implicit conversion to a common type is accepted.
+template <typename EnumTy1, typename EnumTy2,
+          typename UT1 = std::enable_if_t<std::is_enum<EnumTy1>::value,
+                                          std::underlying_type_t<EnumTy1>>,
+          typename UT2 = std::enable_if_t<std::is_enum<EnumTy2>::value,
+                                          std::underlying_type_t<EnumTy2>>>
+constexpr auto addEnumValues(EnumTy1 LHS, EnumTy2 RHS) {
+  return static_cast<UT1>(LHS) + static_cast<UT2>(RHS);
+}
+
 //===----------------------------------------------------------------------===//
 //     Extra additions to <iterator>
 //===----------------------------------------------------------------------===//
Index: llvm/unittests/ADT/STLExtrasTest.cpp
===================================================================
--- llvm/unittests/ADT/STLExtrasTest.cpp
+++ llvm/unittests/ADT/STLExtrasTest.cpp
@@ -9,6 +9,7 @@
 #include "llvm/ADT/STLExtras.h"
 #include "gtest/gtest.h"
 
+#include <climits>
 #include <list>
 #include <vector>
 
@@ -989,4 +990,32 @@
   static_assert(!is_contained({1, 2, 3, 4}, 5), "It's not there :(");
 }
 
+TEST(STLExtrasTest, addEnumValues) {
+  enum A { Zero = 0, One = 1 };
+  enum B { IntMax = INT_MAX, ULongLongMax = ULLONG_MAX };
+  enum class C : unsigned { Two = 2 };
+
+  // Non-fixed underlying types, with same underlying types
+  static_assert(addEnumValues(Zero, One) == 1,
+                "addEnumValues(Zero, One) failed.");
+  static_assert(addEnumValues(IntMax, ULongLongMax) ==
+                    INT_MAX + static_cast<unsigned long long>(ULLONG_MAX),
+                "addEnumValues(IntMax, ULongLongMax) failed.");
+  // Non-fixed underlying types, with different underlying types
+  static_assert(addEnumValues(Zero, IntMax) == INT_MAX,
+                "addEnumValues(Zero, IntMax) failed.");
+  static_assert(addEnumValues(One, ULongLongMax) ==
+                    1 + static_cast<unsigned long long>(ULLONG_MAX),
+                "addEnumValues(One, ULongLongMax) failed.");
+  // Non-fixed underlying type enum and fixed underlying type enum, with same
+  // underlying types
+  static_assert(addEnumValues(One, C::Two) == 3,
+                "addEnumValues(One, C::Two) failed.");
+  // Non-fixed underlying type enum and fixed underlying type enum, with
+  // different underlying types
+  static_assert(addEnumValues(ULongLongMax, C::Two) ==
+                    static_cast<unsigned long long>(ULLONG_MAX) + 2,
+                "addEnumValues(ULongLongMax, C::Two) failed.");
+}
+
 } // namespace