message: Added missing std:: qualifier
diff: # include <string>
  # include <stdexcept>
  -# define TR2_OPTIONAL_REQUIRES(...) typename enable_if<__VA_ARGS__::value, bool>::type = false
  +# define TR2_OPTIONAL_REQUIRES(...) typename std::enable_if<__VA_ARGS__::value, bool>::type = false
  # if defined __GNUC__ // NOTE: GNUC is also defined for Clang
  # if (__GNUC__ == 4) && (__GNUC_MINOR__ >= 8)
  @@ -256,9 +256,9 @@ constexpr nullopt_t nullopt{nullopt_t::init()};
  // 20.5.8, class bad_optional_access
  -class bad_optional_access : public logic_error {
  +class bad_optional_access : public std::logic_error {
  public:
  - explicit bad_optional_access(const string& what_arg) : logic_error{what_arg} {}
  + explicit bad_optional_access(const std::string& what_arg) : logic_error{what_arg} {}
  explicit bad_optional_access(const char* what_arg) : logic_error{what_arg} {}
  };
  @@ -459,9 +459,9 @@ public:
  template <class U>
  auto operator=(U&& v)
  - -> typename enable_if
  + -> typename std::enable_if
  <
  - is_same<typename decay<U>::type, T>::value,
  + std::is_same<typename std::decay<U>::type, T>::value,
  optional&
  >::type
  {
  @@ -479,7 +479,7 @@ public:
  }
  template <class U, class... Args>
  - void emplace(initializer_list<U> il, Args&&... args)
  + void emplace(std::initializer_list<U> il, Args&&... args)
  {
  clear();
  initialize<U, Args...>(il, std::forward<Args>(args)...);
  @@ -647,9 +647,9 @@ public:
  template <typename U>
  auto operator=(U&& rhs) noexcept
  - -> typename enable_if
  + -> typename std::enable_if
  <
  - is_same<typename decay<U>::type, optional<T&>>::value,
  + std::is_same<typename std::decay<U>::type, optional<T&>>::value,
  optional&
  >::type
  {
  @@ -659,9 +659,9 @@ public:
  template <typename U>
  auto operator=(U&& rhs) noexcept
  - -> typename enable_if
  + -> typename std::enable_if
  <
  - !is_same<typename decay<U>::type, optional<T&>>::value,
  + !std::is_same<typename std::decay<U>::type, optional<T&>>::value,
  optional&
  >::type
  = delete;
  @@ -700,9 +700,9 @@ public:
  }
  template <class V>
  - constexpr typename decay<T>::type value_or(V&& v) const
  + constexpr typename std::decay<T>::type value_or(V&& v) const
  {
  - return *this ? **this : detail_::convert<typename decay<T>::type>(constexpr_forward<V>(v));
  + return *this ? **this : detail_::convert<typename std::decay<T>::type>(constexpr_forward<V>(v));
  }
  // x.x.x.x, modifiers
  @@ -1006,13 +1006,13 @@ void swap(optional<T>& x, optional<T>& y) noexcept(noexcept(x.swap(y)))
  template <class T>
  -constexpr optional<typename decay<T>::type> make_optional(T&& v)
  +constexpr optional<typename std::decay<T>::type> make_optional(T&& v)
  {
  - return optional<typename decay<T>::type>(constexpr_forward<T>(v));
  + return optional<typename std::decay<T>::type>(constexpr_forward<T>(v));
  }
  template <class X>
  -constexpr optional<X&> make_optional(reference_wrapper<X> v)
  +constexpr optional<X&> make_optional(std::reference_wrapper<X> v)
  {
  return optional<X&>(v.get());
  }
  