# 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

include/nlohmann/detail/input/json_sax.hpp

@@ -353,7 +353,7 @@ class json_sax_dom_callback_parser
     using parse_event_t = typename BasicJsonType::parse_event_t;
 
     json_sax_dom_callback_parser(BasicJsonType& r,
-                                 const parser_callback_t cb,
+                                 parser_callback_t cb,
                                  const bool allow_exceptions_ = true)
         : root(r), callback(std::move(cb)), allow_exceptions(allow_exceptions_)
     {

include/nlohmann/detail/input/lexer.hpp

@@ -967,7 +967,7 @@ class lexer : public lexer_base<BasicJsonType>
           locale's decimal point is used instead of `.` to work with the
           locale-dependent converters.
     */
-    token_type scan_number()  // lgtm [cpp/use-of-goto]
+    token_type scan_number()  // lgtm [cpp/use-of-goto] `goto` is used in this function to implement the number-parsing state machine described above. By design, any finite input will eventually reach the "done" state or return token_type::parse_error. In each intermediate state, 1 byte of the input is appended to the token_buffer vector, and only the already initialized variables token_buffer, number_type, and error_message are manipulated.
     {
         // reset token_buffer to store the number's bytes
         reset();

include/nlohmann/detail/input/parser.hpp

@@ -69,10 +69,10 @@ class parser
   public:
     /// a parser reading from an input adapter
     explicit parser(InputAdapterType&& adapter,
-                    const parser_callback_t<BasicJsonType>& cb = nullptr,
+                    parser_callback_t<BasicJsonType>& cb = nullptr,
                     const bool allow_exceptions_ = true,
                     const bool skip_comments = false)
-        : callback(cb)
+        : callback(std::move(cb))
         , m_lexer(std::move(adapter), skip_comments)
         , allow_exceptions(allow_exceptions_)
     {

include/nlohmann/detail/iterators/iteration_proxy.hpp

@@ -9,7 +9,7 @@
 #pragma once
 
 #include <cstddef> // size_t
-#include <iterator> // input_iterator_tag
+#include <iterator> // forward_iterator_tag
 #include <string> // string, to_string
 #include <tuple> // tuple_size, get, tuple_element
 #include <utility> // move
@@ -40,7 +40,7 @@ template<typename IteratorType> class iteration_proxy_value
     using value_type = iteration_proxy_value;
     using pointer = value_type *;
     using reference = value_type &;
-    using iterator_category = std::input_iterator_tag;
+    using iterator_category = std::forward_iterator_tag;
     using string_type = typename std::remove_cv< typename std::remove_reference<decltype( std::declval<IteratorType>().key() ) >::type >::type;
 
   private:

include/nlohmann/detail/output/binary_writer.hpp

@@ -612,7 +612,7 @@ class binary_writer
             case value_t::binary:
             {
                 // step 0: determine if the binary type has a set subtype to
-                // determine whether or not to use the ext or fixext types
+                // determine whether to use the ext or fixext types
                 const bool use_ext = j.m_data.m_value.binary->has_subtype();
 
                 // step 1: write control byte and the byte string length