diff --git a/llvm/include/llvm/ADT/PointerUnion.h b/llvm/include/llvm/ADT/PointerUnion.h
--- a/llvm/include/llvm/ADT/PointerUnion.h
+++ b/llvm/include/llvm/ADT/PointerUnion.h
@@ -64,21 +64,6 @@
     return std::min<int>({PointerLikeTypeTraits<Ts>::NumLowBitsAvailable...});
   }
 
-  /// Find the index of a type in a list of types. TypeIndex<T, Us...>::Index
-  /// is the index of T in Us, or sizeof...(Us) if T does not appear in the
-  /// list.
-  template <typename T, typename ...Us> struct TypeIndex;
-  template <typename T, typename ...Us> struct TypeIndex<T, T, Us...> {
-    static constexpr int Index = 0;
-  };
-  template <typename T, typename U, typename... Us>
-  struct TypeIndex<T, U, Us...> {
-    static constexpr int Index = 1 + TypeIndex<T, Us...>::Index;
-  };
-  template <typename T> struct TypeIndex<T> {
-    static constexpr int Index = 0;
-  };
-
   /// Find the first type in a list of types.
   template <typename T, typename...> struct GetFirstType {
     using type = T;
@@ -145,6 +130,7 @@
 ///    P = (float*)0;
 ///    Y = P.get<float*>();   // ok.
 ///    X = P.get<int*>();     // runtime assertion failure.
+///    PointerUnion<int*, int*> Q; // compile time failure.
 template <typename... PTs>
 class PointerUnion
     : public pointer_union_detail::PointerUnionMembers<
@@ -153,12 +139,14 @@
               void *, pointer_union_detail::bitsRequired(sizeof...(PTs)), int,
               pointer_union_detail::PointerUnionUIntTraits<PTs...>>,
           0, PTs...> {
+  static_assert(TypesAreDistinct<PTs...>(),
+                "PointerUnion alternative types cannot be repeated");
   // The first type is special because we want to directly cast a pointer to a
   // default-initialized union to a pointer to the first type. But we don't
   // want PointerUnion to be a 'template <typename First, typename ...Rest>'
   // because it's much more convenient to have a name for the whole pack. So
   // split off the first type here.
-  using First = typename pointer_union_detail::GetFirstType<PTs...>::type;
+  using First = TypeAtIndex<0, PTs...>;
   using Base = typename PointerUnion::PointerUnionMembers;
 
 public:
@@ -175,10 +163,7 @@
 
   /// Test if the Union currently holds the type matching T.
   template <typename T> bool is() const {
-    constexpr int Index = pointer_union_detail::TypeIndex<T, PTs...>::Index;
-    static_assert(Index < sizeof...(PTs),
-                  "PointerUnion::is<T> given type not in the union");
-    return this->Val.getInt() == Index;
+    return this->Val.getInt() == FirstIndexOfType<T, PTs...>{};
   }
 
   /// Returns the value of the specified pointer type.
diff --git a/llvm/include/llvm/ADT/STLExtras.h b/llvm/include/llvm/ADT/STLExtras.h
--- a/llvm/include/llvm/ADT/STLExtras.h
+++ b/llvm/include/llvm/ADT/STLExtras.h
@@ -144,6 +144,60 @@
 struct function_traits<ReturnType (&)(Args...), false>
     : public function_traits<ReturnType (*)(Args...)> {};
 
+namespace detail {
+template <typename T, typename... Ts>
+constexpr size_t getFirstIndexOfTypeImpl() {
+  constexpr bool Matches[] = {false, std::is_same<T, Ts>{}...};
+  // TODO: use llvm::find once it's constexpr (std::find is constexpr in C++20)
+  for (size_t I = 1; I <= sizeof...(Ts); ++I)
+    if (Matches[I])
+      return I - 1;
+  return sizeof...(Ts);
+}
+template <typename T, typename... Ts> constexpr size_t getFirstIndexOfType() {
+  constexpr size_t Index = getFirstIndexOfTypeImpl<T, Ts...>();
+  static_assert(Index != sizeof...(Ts),
+                "type T is not present in type pack Ts");
+  return Index;
+}
+template <typename... Ts> static constexpr bool areTypesDistinct() {
+  constexpr size_t IndexOf[] = {0, getFirstIndexOfType<Ts, Ts...>()...};
+  // TODO: use llvm::find once it's constexpr (std::find is constexpr in C++20)
+  for (size_t I = 1; I <= sizeof...(Ts); ++I)
+    if (I - 1 != IndexOf[I])
+      return false;
+  return true;
+}
+} // namespace detail
+
+/// Determine if all types in Ts are distinct.
+///
+/// Useful to statically assert when Ts is intended to describe a non-multi set
+/// of types.
+template <typename... Ts>
+using TypesAreDistinct =
+    std::integral_constant<bool, detail::areTypesDistinct<Ts...>()>;
+
+/// Find the first index where a type appears in a list of types.
+///
+/// FirstIndexOfType<T, Ts...>::value is the first index of T in Ts.
+///
+/// Typically only meaningful when it is otherwise statically known that the
+/// type pack has no duplicate types. This should be guaranteed explicitly with
+/// static_assert(TypesAreDistinct<Ts...>{}).
+///
+/// It is a compile-time error to instantiate when T is not present in Ts, i.e.
+/// if is_one_of<T, Ts...>::value is false.
+template <typename T, typename... Ts>
+using FirstIndexOfType =
+    std::integral_constant<size_t, detail::getFirstIndexOfType<T, Ts...>()>;
+
+/// Find the type at a given index in a list of types.
+///
+/// TypeAtIndex<I, Ts...> is the type at index I in Ts.
+template <size_t I, typename... Ts>
+using TypeAtIndex = std::tuple_element_t<I, std::tuple<Ts...>>;
+
 //===----------------------------------------------------------------------===//
 //     Extra additions to <functional>
 //===----------------------------------------------------------------------===//
diff --git a/llvm/unittests/ADT/STLExtrasTest.cpp b/llvm/unittests/ADT/STLExtrasTest.cpp
--- a/llvm/unittests/ADT/STLExtrasTest.cpp
+++ b/llvm/unittests/ADT/STLExtrasTest.cpp
@@ -711,4 +711,40 @@
   EXPECT_EQ(V4.size(), 4U);
   EXPECT_TRUE(llvm::all_of(V4, HasVal));
 }
+
+TEST(STLExtrasTest, TypesAreDistinct) {
+  EXPECT_TRUE((llvm::TypesAreDistinct<>::value));
+  EXPECT_TRUE((llvm::TypesAreDistinct<int>::value));
+  EXPECT_FALSE((llvm::TypesAreDistinct<int, int>::value));
+  EXPECT_TRUE((llvm::TypesAreDistinct<int, float>::value));
+  EXPECT_FALSE((llvm::TypesAreDistinct<int, float, int>::value));
+  EXPECT_TRUE((llvm::TypesAreDistinct<int, float, double>::value));
+  EXPECT_FALSE((llvm::TypesAreDistinct<int, float, double, float>::value));
+  EXPECT_TRUE((llvm::TypesAreDistinct<int, int *>::value));
+  EXPECT_TRUE((llvm::TypesAreDistinct<int, int &>::value));
+  EXPECT_TRUE((llvm::TypesAreDistinct<int, int &&>::value));
+  EXPECT_TRUE((llvm::TypesAreDistinct<int, const int>::value));
+}
+
+TEST(STLExtrasTest, FirstIndexOfType) {
+  EXPECT_EQ((llvm::FirstIndexOfType<int, int>::value), 0u);
+  EXPECT_EQ((llvm::FirstIndexOfType<int, int, int>::value), 0u);
+  EXPECT_EQ((llvm::FirstIndexOfType<int, float, int>::value), 1u);
+  EXPECT_EQ((llvm::FirstIndexOfType<int const *, float, int, int const *,
+                                    const int>::value),
+            2u);
+}
+
+TEST(STLExtrasTest, TypeAtIndex) {
+  EXPECT_TRUE((std::is_same<int, llvm::TypeAtIndex<0, int>>::value));
+  EXPECT_TRUE((std::is_same<int, llvm::TypeAtIndex<0, int, float>>::value));
+  EXPECT_TRUE((std::is_same<float, llvm::TypeAtIndex<1, int, float>>::value));
+  EXPECT_TRUE(
+      (std::is_same<float, llvm::TypeAtIndex<1, int, float, double>>::value));
+  EXPECT_TRUE(
+      (std::is_same<float, llvm::TypeAtIndex<1, int, float, double>>::value));
+  EXPECT_TRUE(
+      (std::is_same<double, llvm::TypeAtIndex<2, int, float, double>>::value));
+}
+
 } // namespace