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
@@ -16,6 +16,7 @@
 
 #include "llvm/ADT/DenseMapInfo.h"
 #include "llvm/ADT/PointerIntPair.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/PointerLikeTypeTraits.h"
 #include <algorithm>
 #include <cassert>
@@ -35,21 +36,6 @@
     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;
@@ -116,6 +102,7 @@
 ///    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<
@@ -124,12 +111,14 @@
               void *, pointer_union_detail::bitsRequired(sizeof...(PTs)), int,
               pointer_union_detail::PointerUnionUIntTraits<PTs...>>,
           0, PTs...> {
+  static_assert(TypesAreDistinct<PTs...>::value,
+                "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:
@@ -146,10 +135,7 @@
 
   /// 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...>::value;
   }
 
   /// 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 +144,61 @@
 struct function_traits<ReturnType (&)(Args...), false>
     : public function_traits<ReturnType (*)(Args...)> {};
 
+/// traits class for checking whether type T is one of any of the given
+/// types in the variadic list.
+template <typename T, typename... Ts>
+using is_one_of = disjunction<std::is_same<T, Ts>...>;
+
+/// traits class for checking whether type T is a base class for all
+///  the given types in the variadic list.
+template <typename T, typename... Ts>
+using are_base_of = conjunction<std::is_base_of<T, Ts>...>;
+
+namespace detail {
+template <typename T, typename... Us> struct TypesAreDistinct;
+template <typename T, typename... Us>
+struct TypesAreDistinct
+    : std::integral_constant<bool, !is_one_of<T, Us...>::value &&
+                                       TypesAreDistinct<Us...>::value> {};
+template <typename T> struct TypesAreDistinct<T> : std::true_type {};
+} // 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.
+///
+/// Expensive (currently quadratic in sizeof(Ts...)), and so should only be
+/// asserted once per instantiation of a type which requires it.
+template <typename... Ts> struct TypesAreDistinct;
+template <> struct TypesAreDistinct<> : std::true_type {};
+template <typename... Ts>
+struct TypesAreDistinct
+    : std::integral_constant<bool, detail::TypesAreDistinct<Ts...>::value> {};
+
+/// Find the first index where a type appears in a list of types.
+///
+/// FirstIndexOfType<T, Us...>::value is the first index of T in Us.
+///
+/// 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<Us...>::value).
+///
+/// It is a compile-time error to instantiate when T is not present in Us, i.e.
+/// if is_one_of<T, Us...>::value is false.
+template <typename T, typename... Us> struct FirstIndexOfType;
+template <typename T, typename U, typename... Us>
+struct FirstIndexOfType<T, U, Us...>
+    : std::integral_constant<size_t, 1 + FirstIndexOfType<T, Us...>::value> {};
+template <typename T, typename... Us>
+struct FirstIndexOfType<T, T, Us...> : std::integral_constant<size_t, 0> {};
+
+/// 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
@@ -905,4 +905,39 @@
             all_of_zip(v1, v2, v_short, [](int, int, int) { return true; }));
 }
 
+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