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diff --git a/Firestore/core/src/firebase/firestore/util/type_traits.h b/Firestore/core/src/firebase/firestore/util/type_traits.h
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+/*
+ * Copyright 2018 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_TYPE_TRAITS_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_TYPE_TRAITS_H_
+
+#if __OBJC__
+#import <objc/objc.h>  // for id
+#endif
+
+#include <type_traits>
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+#if __OBJC__
+
+/**
+ * A type trait that identifies whether or not the given pointer points to an
+ * Objective-C object.
+ *
+ * is_objective_c_pointer<NSObject*>::value == true
+ * is_objective_c_pointer<NSArray<NSString*>*>::value == true
+ *
+ * // id is a dynamically typed pointer to an Objective-C object.
+ * is_objective_c_pointer<id>::value == true
+ *
+ * // pointers to C++ classes are not Objective-C pointers.
+ * is_objective_c_pointer<void*>::value == false
+ * is_objective_c_pointer<std::string*>::value == false
+ * is_objective_c_pointer<std::unique_ptr<int>>::value == false
+ */
+template <typename T>
+struct is_objective_c_pointer {
+ private:
+  using yes_type = char (&)[10];
+  using no_type = char (&)[1];
+
+  /**
+   * A non-existent function declared to produce a pointer to type T (which is
+   * consistent with the way Objective-C objects are referenced).
+   *
+   * Note that there is no definition for this function but that's okay because
+   * we only need it to reason about the function's type at compile type.
+   */
+  static T Pointer();
+
+  static yes_type Choose(id value);
+  static no_type Choose(...);
+
+ public:
+  using value_type = bool;
+
+  enum { value = sizeof(Choose(Pointer())) == sizeof(yes_type) };
+
+  constexpr operator bool() const {
+    return value;
+  }
+
+  constexpr bool operator()() const {
+    return value;
+  }
+};
+
+// Hard-code the answer for `void` because you can't pass arguments of type
+// `void` to another function.
+template <>
+struct is_objective_c_pointer<void> : public std::false_type {};
+
+#endif  // __OBJC__
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_TYPE_TRAITS_H_