diff --git a/include/nlohmann/detail/conversions/from_json.hpp b/include/nlohmann/detail/conversions/from_json.hpp
index d647d7423..797f714df 100644
--- a/include/nlohmann/detail/conversions/from_json.hpp
+++ b/include/nlohmann/detail/conversions/from_json.hpp
@@ -13,9 +13,6 @@
 #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
@@ -32,6 +29,11 @@
 #include <nlohmann/detail/string_concat.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
+// include after macro_scope.hpp
+#ifdef JSON_HAS_CPP_17
+    #include <optional> // optional
+#endif
+
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
@@ -47,7 +49,6 @@ inline void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
 }
 
 #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)
 {
@@ -60,8 +61,6 @@ void from_json(const BasicJsonType& j, std::optional<T>& opt)
         opt.emplace(j.template get<T>());
     }
 }
-
-#endif // JSON_USE_IMPLICIT_CONVERSIONS
 #endif // JSON_HAS_CPP_17
 
 // overloads for basic_json template parameters
diff --git a/include/nlohmann/detail/conversions/to_json.hpp b/include/nlohmann/detail/conversions/to_json.hpp
index ead45665f..f8413850d 100644
--- a/include/nlohmann/detail/conversions/to_json.hpp
+++ b/include/nlohmann/detail/conversions/to_json.hpp
@@ -267,7 +267,7 @@ struct external_constructor<value_t::object>
 #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)
+void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept
 {
     if (opt.has_value())
     {
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 7d0ffaab1..f4e63aabe 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -173,9 +173,6 @@
 #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
@@ -4817,6 +4814,11 @@ NLOHMANN_JSON_NAMESPACE_END
 // #include <nlohmann/detail/value_t.hpp>
 
 
+// include after macro_scope.hpp
+#ifdef JSON_HAS_CPP_17
+    #include <optional> // optional
+#endif
+
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
@@ -4832,7 +4834,6 @@ inline void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
 }
 
 #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)
 {
@@ -4845,8 +4846,6 @@ void from_json(const BasicJsonType& j, std::optional<T>& opt)
         opt.emplace(j.template get<T>());
     }
 }
-
-#endif // JSON_USE_IMPLICIT_CONVERSIONS
 #endif // JSON_HAS_CPP_17
 
 // overloads for basic_json template parameters
@@ -5914,7 +5913,7 @@ struct external_constructor<value_t::object>
 #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)
+void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept
 {
     if (opt.has_value())
     {
diff --git a/tests/src/unit-regression2.cpp b/tests/src/unit-regression2.cpp
index 2c3977fef..22bb8efc6 100644
--- a/tests/src/unit-regression2.cpp
+++ b/tests/src/unit-regression2.cpp
@@ -36,6 +36,14 @@ using ordered_json = nlohmann::ordered_json;
     #include <variant>
 #endif
 
+#ifdef JSON_HAS_CPP_17
+    #if __has_include(<optional>)
+        #include <optional>
+    #elif __has_include(<experimental/optional>)
+        #include <experimental/optional>
+    #endif
+#endif
+
 #ifdef JSON_HAS_CPP_20
     #if __has_include(<span>)
         #include <span>
@@ -390,6 +398,19 @@ struct Example_3810
 
 NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(Example_3810, bla); // NOLINT(misc-use-internal-linkage)
 
+/////////////////////////////////////////////////////////////////////
+// for #4740
+/////////////////////////////////////////////////////////////////////
+
+#ifdef JSON_HAS_CPP_17
+struct Example_4740
+{
+    std::optional<std::string> host = std::nullopt;
+    std::optional<int> port = std::nullopt;
+    NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(Example_4740, host, port)
+};
+#endif
+
 TEST_CASE("regression tests 2")
 {
     SECTION("issue #1001 - Fix memory leak during parser callback")
@@ -1037,6 +1058,28 @@ TEST_CASE("regression tests 2")
         oj["test"] = states;
         CHECK(oj["test"].dump() == expected);
     }
+
+#ifdef JSON_HAS_CPP_17
+    SECTION("issue #4740 - build issue with std::optional")
+    {
+        const auto t1 = Example_4740();
+        const auto j1 = nlohmann::json(t1);
+        CHECK(j1.dump() == "{\"host\":null,\"port\":null}");
+        const auto t2 = j1.get<Example_4740>();
+        CHECK(!t2.host.has_value());
+        CHECK(!t2.port.has_value());
+
+        // improve coverage
+        auto t3 = Example_4740();
+        t3.port = 80;
+        t3.host = "example.com";
+        const auto j2 = nlohmann::json(t3);
+        CHECK(j2.dump() == "{\"host\":\"example.com\",\"port\":80}");
+        const auto t4 = j2.get<Example_4740>();
+        CHECK(t4.host.has_value());
+        CHECK(t4.port.has_value());
+    }
+#endif
 }
 
 DOCTEST_CLANG_SUPPRESS_WARNING_POP