# Conflicts:

#	.github/workflows/macos.yml
#	include/nlohmann/detail/input/parser.hpp
#	include/nlohmann/detail/meta/std_fs.hpp
#	include/nlohmann/json.hpp
#	single_include/nlohmann/json.hpp

include/nlohmann/detail/conversions/from_json.hpp

@@ -13,6 +13,9 @@
 #include <forward_list> // forward_list
 #include <iterator> // inserter, front_inserter, end
 #include <map> // map
+#ifdef JSON_HAS_CPP_17
+    #include <optional> // optional
+#endif
 #include <string> // string
 #include <tuple> // tuple, make_tuple
 #include <type_traits> // is_arithmetic, is_same, is_enum, underlying_type, is_convertible
@@ -20,6 +23,7 @@
 #include <utility> // pair, declval
 #include <valarray> // valarray
 
+
 #include <nlohmann/detail/exceptions.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/meta/cpp_future.hpp>
@@ -43,6 +47,24 @@ inline void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
     n = nullptr;
 }
 
+#ifdef JSON_HAS_CPP_17
+#ifndef JSON_USE_IMPLICIT_CONVERSIONS
+template<typename BasicJsonType, typename T>
+void from_json(const BasicJsonType& j, std::optional<T>& opt)
+{
+    if (j.is_null())
+    {
+        opt = std::nullopt;
+    }
+    else
+    {
+        opt.emplace(j.template get<T>());
+    }
+}
+
+#endif // JSON_USE_IMPLICIT_CONVERSIONS
+#endif // JSON_HAS_CPP_17
+
 // overloads for basic_json template parameters
 template < typename BasicJsonType, typename ArithmeticType,
            enable_if_t < std::is_arithmetic<ArithmeticType>::value&&

include/nlohmann/detail/conversions/to_json.hpp

@@ -10,6 +10,9 @@
 
 #include <algorithm> // copy
 #include <iterator> // begin, end
+#ifdef JSON_HAS_CPP_17
+    #include <optional> // optional
+#endif
 #include <string> // string
 #include <tuple> // tuple, get
 #include <type_traits> // is_same, is_constructible, is_floating_point, is_enum, underlying_type
@@ -260,6 +263,22 @@ struct external_constructor<value_t::object>
 // to_json //
 /////////////
 
+#ifdef JSON_HAS_CPP_17
+template<typename BasicJsonType, typename T,
+         enable_if_t<std::is_constructible<BasicJsonType, T>::value, int> = 0>
+void to_json(BasicJsonType& j, const std::optional<T>& opt)
+{
+    if (opt.has_value())
+    {
+        j = *opt;
+    }
+    else
+    {
+        j = nullptr;
+    }
+}
+#endif
+
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_same<T, typename BasicJsonType::boolean_t>::value, int> = 0>
 inline void to_json(BasicJsonType& j, T b) noexcept