Forward-declare tuple in utility.h and define it in tuple.h

This is to:
1. Fix compile error due to duplicate definition, and
2. Reflect how it is the real C++ standard library.

Author: jeongsoolee09

cpp/common/test/includes/standard-library/tuple.h

@@ -1,11 +1,12 @@
 namespace std {
-template <class... Types> class tuple {};
+// Definition
+template <typename... T1> class tuple {};
+
 template <class... Types> std::tuple<Types...> make_tuple(Types &&...args);
 struct ignore_t {
   template <typename T>
   constexpr // required since C++14
-      void
-      operator=(T &&) const noexcept {}
+      void operator=(T &&) const noexcept {}
 };
 inline const std::ignore_t ignore; // 'const' only until C++17
 } // namespace std

cpp/common/test/includes/standard-library/utility.h

@@ -9,24 +9,24 @@ template <class T> typename add_rvalue_reference<T>::type declval() noexcept;
 
 template <class T> void swap(T &a, T &b) noexcept;
 
-template<class... Types>
-struct tuple {};
+// Forward declaration only - defined in <tuple>
+template <typename... T1> class tuple;
 
 template <class T, class U> struct pair : tuple<T, U> {
-    T first;
-    U second;
-    pair(T t, U u);
+  T first;
+  U second;
+  pair(T t, U u);
 };
 template <class T, class U> std::pair<T, U> make_pair(T &&x, U &&y);
 
-template<size_t N, class T, class U>
+template <size_t N, class T, class U>
 constexpr auto get(const std::pair<T, U> &p) noexcept {
-    if constexpr (N == 0) {
-        return p.first;
-    } else if constexpr (N == 1) {
-        return p.second;
-    } else {
-        static_assert(N < 2, "Index out of bounds for pair");
-    }
+  if constexpr (N == 0) {
+    return p.first;
+  } else if constexpr (N == 1) {
+    return p.second;
+  } else {
+    static_assert(N < 2, "Index out of bounds for pair");
+  }
 }
-} // namespace std
+} // namespace std