upgrade variant dependency

ThirdParty/variant/optional.hpp

@@ -0,0 +1,78 @@
+#ifndef MAPBOX_UTIL_OPTIONAL_HPP
+#define MAPBOX_UTIL_OPTIONAL_HPP
+
+#include <type_traits>
+
+#include "variant.hpp"
+
+namespace mapbox
+{
+namespace util
+{
+
+template <typename T> class optional
+{
+    static_assert(!std::is_reference<T>::value, "optional doesn't support references");
+
+    struct none_type
+    {
+    };
+
+    variant<none_type, T> variant_;
+
+  public:
+    optional() = default;
+
+    optional(optional const &rhs)
+    {
+        if (this != &rhs)
+        { // protect against invalid self-assignment
+            variant_ = rhs.variant_;
+        }
+    }
+
+    optional(T const &v) { variant_ = v; }
+
+    optional &operator=(optional other)
+    { // note: argument passed by value!
+        if (this != &other)
+        {
+            swap(other);
+        }
+        return *this;
+    }
+
+    explicit operator bool() const noexcept { return variant_.template is<T>(); }
+
+    T const &get() const { return variant_.template get<T>(); }
+    T &get() { return variant_.template get<T>(); }
+
+    T const &operator*() const { return this->get(); }
+    T operator*() { return this->get(); }
+
+    optional &operator=(T const &v)
+    {
+        variant_ = v;
+        return *this;
+    }
+
+    optional &operator=(optional const &rhs)
+    {
+        if (this != &rhs)
+        {
+            variant_ = rhs.variant_;
+        }
+        return *this;
+    }
+
+    template <typename... Args> void emplace(Args &&... args)
+    {
+        variant_ = T{std::forward<Args>(args)...};
+    }
+
+    void reset() { variant_ = none_type{}; }
+};
+}
+}
+
+#endif