diff --git a/include/nlohmann/detail/conversions/from_json.hpp b/include/nlohmann/detail/conversions/from_json.hpp
index 079baa723d..f0a93f2918 100644
--- a/include/nlohmann/detail/conversions/from_json.hpp
+++ b/include/nlohmann/detail/conversions/from_json.hpp
@@ -39,7 +39,7 @@ namespace nlohmann
 namespace detail
 {
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
+inline void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_null()))
     {
@@ -86,7 +86,7 @@ void get_arithmetic_value(const BasicJsonType& j, ArithmeticType& val)
 }
 
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, typename BasicJsonType::boolean_t& b)
+inline void from_json(const BasicJsonType& j, typename BasicJsonType::boolean_t& b)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_boolean()))
     {
@@ -96,7 +96,7 @@ void from_json(const BasicJsonType& j, typename BasicJsonType::boolean_t& b)
 }
 
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, typename BasicJsonType::string_t& s)
+inline void from_json(const BasicJsonType& j, typename BasicJsonType::string_t& s)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_string()))
     {
@@ -111,7 +111,7 @@ template <
         std::is_assignable<StringType&, const typename BasicJsonType::string_t>::value
         && !std::is_same<typename BasicJsonType::string_t, StringType>::value
         && !is_json_ref<StringType>::value, int > = 0 >
-void from_json(const BasicJsonType& j, StringType& s)
+inline void from_json(const BasicJsonType& j, StringType& s)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_string()))
     {
@@ -122,26 +122,26 @@ void from_json(const BasicJsonType& j, StringType& s)
 }
 
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, typename BasicJsonType::number_float_t& val)
+inline void from_json(const BasicJsonType& j, typename BasicJsonType::number_float_t& val)
 {
     get_arithmetic_value(j, val);
 }
 
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, typename BasicJsonType::number_unsigned_t& val)
+inline void from_json(const BasicJsonType& j, typename BasicJsonType::number_unsigned_t& val)
 {
     get_arithmetic_value(j, val);
 }
 
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, typename BasicJsonType::number_integer_t& val)
+inline void from_json(const BasicJsonType& j, typename BasicJsonType::number_integer_t& val)
 {
     get_arithmetic_value(j, val);
 }
 
 template<typename BasicJsonType, typename EnumType,
          enable_if_t<std::is_enum<EnumType>::value, int> = 0>
-void from_json(const BasicJsonType& j, EnumType& e)
+inline void from_json(const BasicJsonType& j, EnumType& e)
 {
     typename std::underlying_type<EnumType>::type val;
     get_arithmetic_value(j, val);
@@ -151,7 +151,7 @@ void from_json(const BasicJsonType& j, EnumType& e)
 // forward_list doesn't have an insert method
 template<typename BasicJsonType, typename T, typename Allocator,
          enable_if_t<is_getable<BasicJsonType, T>::value, int> = 0>
-void from_json(const BasicJsonType& j, std::forward_list<T, Allocator>& l)
+inline void from_json(const BasicJsonType& j, std::forward_list<T, Allocator>& l)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
@@ -168,7 +168,7 @@ void from_json(const BasicJsonType& j, std::forward_list<T, Allocator>& l)
 // valarray doesn't have an insert method
 template<typename BasicJsonType, typename T,
          enable_if_t<is_getable<BasicJsonType, T>::value, int> = 0>
-void from_json(const BasicJsonType& j, std::valarray<T>& l)
+inline void from_json(const BasicJsonType& j, std::valarray<T>& l)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
@@ -193,7 +193,7 @@ auto from_json(const BasicJsonType& j, T (&arr)[N])  // NOLINT(cppcoreguidelines
 }
 
 template<typename BasicJsonType>
-void from_json_array_impl(const BasicJsonType& j, typename BasicJsonType::array_t& arr, priority_tag<3> /*unused*/)
+inline void from_json_array_impl(const BasicJsonType& j, typename BasicJsonType::array_t& arr, priority_tag<3> /*unused*/)
 {
     arr = *j.template get_ptr<const typename BasicJsonType::array_t*>();
 }
@@ -237,8 +237,8 @@ template<typename BasicJsonType, typename ConstructibleArrayType,
          enable_if_t<
              std::is_assignable<ConstructibleArrayType&, ConstructibleArrayType>::value,
              int> = 0>
-void from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr,
-                          priority_tag<0> /*unused*/)
+inline void from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr,
+                                 priority_tag<0> /*unused*/)
 {
     using std::end;
 
@@ -295,7 +295,7 @@ auto from_json(BasicJsonType&& j, identity_tag<std::array<T, N>> tag)
 }
 
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, typename BasicJsonType::binary_t& bin)
+inline void from_json(const BasicJsonType& j, typename BasicJsonType::binary_t& bin)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_binary()))
     {
@@ -307,7 +307,7 @@ void from_json(const BasicJsonType& j, typename BasicJsonType::binary_t& bin)
 
 template<typename BasicJsonType, typename ConstructibleObjectType,
          enable_if_t<is_constructible_object_type<BasicJsonType, ConstructibleObjectType>::value, int> = 0>
-void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
+inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_object()))
     {
@@ -339,7 +339,7 @@ template < typename BasicJsonType, typename ArithmeticType,
                !std::is_same<ArithmeticType, typename BasicJsonType::number_float_t>::value&&
                !std::is_same<ArithmeticType, typename BasicJsonType::boolean_t>::value,
                int > = 0 >
-void from_json(const BasicJsonType& j, ArithmeticType& val)
+inline void from_json(const BasicJsonType& j, ArithmeticType& val)
 {
     switch (static_cast<value_t>(j))
     {
@@ -389,7 +389,7 @@ std::pair<A1, A2> from_json_tuple_impl(BasicJsonType&& j, identity_tag<std::pair
 }
 
 template<typename BasicJsonType, typename A1, typename A2>
-void from_json_tuple_impl(BasicJsonType&& j, std::pair<A1, A2>& p, priority_tag<1> /*unused*/)
+inline void from_json_tuple_impl(BasicJsonType&& j, std::pair<A1, A2>& p, priority_tag<1> /*unused*/)
 {
     p = from_json_tuple_impl(std::forward<BasicJsonType>(j), identity_tag<std::pair<A1, A2>> {}, priority_tag<0> {});
 }
@@ -401,7 +401,7 @@ std::tuple<Args...> from_json_tuple_impl(BasicJsonType&& j, identity_tag<std::tu
 }
 
 template<typename BasicJsonType, typename... Args>
-void from_json_tuple_impl(BasicJsonType&& j, std::tuple<Args...>& t, priority_tag<3> /*unused*/)
+inline void from_json_tuple_impl(BasicJsonType&& j, std::tuple<Args...>& t, priority_tag<3> /*unused*/)
 {
     t = from_json_tuple_impl_base<BasicJsonType, Args...>(std::forward<BasicJsonType>(j), index_sequence_for<Args...> {});
 }
@@ -421,7 +421,7 @@ auto from_json(BasicJsonType&& j, TupleRelated&& t)
 template < typename BasicJsonType, typename Key, typename Value, typename Compare, typename Allocator,
            typename = enable_if_t < !std::is_constructible <
                                         typename BasicJsonType::string_t, Key >::value >>
-void from_json(const BasicJsonType& j, std::map<Key, Value, Compare, Allocator>& m)
+inline void from_json(const BasicJsonType& j, std::map<Key, Value, Compare, Allocator>& m)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
@@ -441,7 +441,7 @@ void from_json(const BasicJsonType& j, std::map<Key, Value, Compare, Allocator>&
 template < typename BasicJsonType, typename Key, typename Value, typename Hash, typename KeyEqual, typename Allocator,
            typename = enable_if_t < !std::is_constructible <
                                         typename BasicJsonType::string_t, Key >::value >>
-void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyEqual, Allocator>& m)
+inline void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyEqual, Allocator>& m)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
@@ -460,7 +460,7 @@ void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyE
 
 #if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, std_fs::path& p)
+inline void from_json(const BasicJsonType& j, std_fs::path& p)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_string()))
     {
@@ -482,11 +482,16 @@ struct from_json_fn
 };
 }  // namespace detail
 
+#ifndef JSON_HAS_CPP_17
 /// namespace to hold default `from_json` function
 /// to see why this is required:
 /// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4381.html
 namespace // NOLINT(cert-dcl59-cpp,fuchsia-header-anon-namespaces,google-build-namespaces)
 {
-constexpr const auto& from_json = detail::static_const<detail::from_json_fn>::value; // NOLINT(misc-definitions-in-headers)
+#endif
+JSON_INLINE_VARIABLE constexpr const auto& from_json = // NOLINT(misc-definitions-in-headers)
+    detail::static_const<detail::from_json_fn>::value;
+#ifndef JSON_HAS_CPP_17
 } // namespace
+#endif
 } // namespace nlohmann
diff --git a/include/nlohmann/detail/conversions/to_json.hpp b/include/nlohmann/detail/conversions/to_json.hpp
index 7628ae0116..fde46ef7c3 100644
--- a/include/nlohmann/detail/conversions/to_json.hpp
+++ b/include/nlohmann/detail/conversions/to_json.hpp
@@ -267,55 +267,55 @@ struct external_constructor<value_t::object>
 
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_same<T, typename BasicJsonType::boolean_t>::value, int> = 0>
-void to_json(BasicJsonType& j, T b) noexcept
+inline void to_json(BasicJsonType& j, T b) noexcept
 {
     external_constructor<value_t::boolean>::construct(j, b);
 }
 
 template<typename BasicJsonType, typename CompatibleString,
          enable_if_t<std::is_constructible<typename BasicJsonType::string_t, CompatibleString>::value, int> = 0>
-void to_json(BasicJsonType& j, const CompatibleString& s)
+inline void to_json(BasicJsonType& j, const CompatibleString& s)
 {
     external_constructor<value_t::string>::construct(j, s);
 }
 
 template<typename BasicJsonType>
-void to_json(BasicJsonType& j, typename BasicJsonType::string_t&& s)
+inline void to_json(BasicJsonType& j, typename BasicJsonType::string_t&& s)
 {
     external_constructor<value_t::string>::construct(j, std::move(s));
 }
 
 template<typename BasicJsonType, typename FloatType,
          enable_if_t<std::is_floating_point<FloatType>::value, int> = 0>
-void to_json(BasicJsonType& j, FloatType val) noexcept
+inline void to_json(BasicJsonType& j, FloatType val) noexcept
 {
     external_constructor<value_t::number_float>::construct(j, static_cast<typename BasicJsonType::number_float_t>(val));
 }
 
 template<typename BasicJsonType, typename CompatibleNumberUnsignedType,
          enable_if_t<is_compatible_integer_type<typename BasicJsonType::number_unsigned_t, CompatibleNumberUnsignedType>::value, int> = 0>
-void to_json(BasicJsonType& j, CompatibleNumberUnsignedType val) noexcept
+inline void to_json(BasicJsonType& j, CompatibleNumberUnsignedType val) noexcept
 {
     external_constructor<value_t::number_unsigned>::construct(j, static_cast<typename BasicJsonType::number_unsigned_t>(val));
 }
 
 template<typename BasicJsonType, typename CompatibleNumberIntegerType,
          enable_if_t<is_compatible_integer_type<typename BasicJsonType::number_integer_t, CompatibleNumberIntegerType>::value, int> = 0>
-void to_json(BasicJsonType& j, CompatibleNumberIntegerType val) noexcept
+inline void to_json(BasicJsonType& j, CompatibleNumberIntegerType val) noexcept
 {
     external_constructor<value_t::number_integer>::construct(j, static_cast<typename BasicJsonType::number_integer_t>(val));
 }
 
 template<typename BasicJsonType, typename EnumType,
          enable_if_t<std::is_enum<EnumType>::value, int> = 0>
-void to_json(BasicJsonType& j, EnumType e) noexcept
+inline void to_json(BasicJsonType& j, EnumType e) noexcept
 {
     using underlying_type = typename std::underlying_type<EnumType>::type;
     external_constructor<value_t::number_integer>::construct(j, static_cast<underlying_type>(e));
 }
 
 template<typename BasicJsonType>
-void to_json(BasicJsonType& j, const std::vector<bool>& e)
+inline void to_json(BasicJsonType& j, const std::vector<bool>& e)
 {
     external_constructor<value_t::array>::construct(j, e);
 }
@@ -328,39 +328,39 @@ template < typename BasicJsonType, typename CompatibleArrayType,
                          !std::is_same<typename BasicJsonType::binary_t, CompatibleArrayType>::value&&
                          !is_basic_json<CompatibleArrayType>::value,
                          int > = 0 >
-void to_json(BasicJsonType& j, const CompatibleArrayType& arr)
+inline void to_json(BasicJsonType& j, const CompatibleArrayType& arr)
 {
     external_constructor<value_t::array>::construct(j, arr);
 }
 
 template<typename BasicJsonType>
-void to_json(BasicJsonType& j, const typename BasicJsonType::binary_t& bin)
+inline void to_json(BasicJsonType& j, const typename BasicJsonType::binary_t& bin)
 {
     external_constructor<value_t::binary>::construct(j, bin);
 }
 
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_convertible<T, BasicJsonType>::value, int> = 0>
-void to_json(BasicJsonType& j, const std::valarray<T>& arr)
+inline void to_json(BasicJsonType& j, const std::valarray<T>& arr)
 {
     external_constructor<value_t::array>::construct(j, std::move(arr));
 }
 
 template<typename BasicJsonType>
-void to_json(BasicJsonType& j, typename BasicJsonType::array_t&& arr)
+inline void to_json(BasicJsonType& j, typename BasicJsonType::array_t&& arr)
 {
     external_constructor<value_t::array>::construct(j, std::move(arr));
 }
 
 template < typename BasicJsonType, typename CompatibleObjectType,
            enable_if_t < is_compatible_object_type<BasicJsonType, CompatibleObjectType>::value&& !is_basic_json<CompatibleObjectType>::value, int > = 0 >
-void to_json(BasicJsonType& j, const CompatibleObjectType& obj)
+inline void to_json(BasicJsonType& j, const CompatibleObjectType& obj)
 {
     external_constructor<value_t::object>::construct(j, obj);
 }
 
 template<typename BasicJsonType>
-void to_json(BasicJsonType& j, typename BasicJsonType::object_t&& obj)
+inline void to_json(BasicJsonType& j, typename BasicJsonType::object_t&& obj)
 {
     external_constructor<value_t::object>::construct(j, std::move(obj));
 }
@@ -370,13 +370,13 @@ template <
     enable_if_t < !std::is_constructible<typename BasicJsonType::string_t,
                   const T(&)[N]>::value, // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
                   int > = 0 >
-void to_json(BasicJsonType& j, const T(&arr)[N]) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
+inline void to_json(BasicJsonType& j, const T(&arr)[N]) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
 {
     external_constructor<value_t::array>::construct(j, arr);
 }
 
 template < typename BasicJsonType, typename T1, typename T2, enable_if_t < std::is_constructible<BasicJsonType, T1>::value&& std::is_constructible<BasicJsonType, T2>::value, int > = 0 >
-void to_json(BasicJsonType& j, const std::pair<T1, T2>& p)
+inline void to_json(BasicJsonType& j, const std::pair<T1, T2>& p)
 {
     j = { p.first, p.second };
 }
@@ -384,26 +384,26 @@ void to_json(BasicJsonType& j, const std::pair<T1, T2>& p)
 // for https://github.com/nlohmann/json/pull/1134
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_same<T, iteration_proxy_value<typename BasicJsonType::iterator>>::value, int> = 0>
-void to_json(BasicJsonType& j, const T& b)
+inline void to_json(BasicJsonType& j, const T& b)
 {
     j = { {b.key(), b.value()} };
 }
 
 template<typename BasicJsonType, typename Tuple, std::size_t... Idx>
-void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<Idx...> /*unused*/)
+inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<Idx...> /*unused*/)
 {
     j = { std::get<Idx>(t)... };
 }
 
 template<typename BasicJsonType, typename T, enable_if_t<is_constructible_tuple<BasicJsonType, T>::value, int > = 0>
-void to_json(BasicJsonType& j, const T& t)
+inline void to_json(BasicJsonType& j, const T& t)
 {
     to_json_tuple_impl(j, t, make_index_sequence<std::tuple_size<T>::value> {});
 }
 
 #if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
 template<typename BasicJsonType>
-void to_json(BasicJsonType& j, const std_fs::path& p)
+inline void to_json(BasicJsonType& j, const std_fs::path& p)
 {
     j = p.string();
 }
@@ -420,11 +420,16 @@ struct to_json_fn
 };
 }  // namespace detail
 
+#ifndef JSON_HAS_CPP_17
 /// namespace to hold default `to_json` function
 /// to see why this is required:
 /// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4381.html
 namespace // NOLINT(cert-dcl59-cpp,fuchsia-header-anon-namespaces,google-build-namespaces)
 {
-constexpr const auto& to_json = detail::static_const<detail::to_json_fn>::value; // NOLINT(misc-definitions-in-headers)
+#endif
+JSON_INLINE_VARIABLE constexpr const auto& to_json = // NOLINT(misc-definitions-in-headers)
+    detail::static_const<detail::to_json_fn>::value;
+#ifndef JSON_HAS_CPP_17
 } // namespace
+#endif
 } // namespace nlohmann
diff --git a/include/nlohmann/detail/macro_scope.hpp b/include/nlohmann/detail/macro_scope.hpp
index ba80c77bc1..18a527c195 100644
--- a/include/nlohmann/detail/macro_scope.hpp
+++ b/include/nlohmann/detail/macro_scope.hpp
@@ -123,6 +123,12 @@
     #endif
 #endif
 
+#ifdef JSON_HAS_CPP_17
+    #define JSON_INLINE_VARIABLE inline
+#else
+    #define JSON_INLINE_VARIABLE
+#endif
+
 #if JSON_HEDLEY_HAS_ATTRIBUTE(no_unique_address)
     #define JSON_NO_UNIQUE_ADDRESS [[no_unique_address]]
 #else
diff --git a/include/nlohmann/detail/macro_unscope.hpp b/include/nlohmann/detail/macro_unscope.hpp
index ed914815a5..396fe1a334 100644
--- a/include/nlohmann/detail/macro_unscope.hpp
+++ b/include/nlohmann/detail/macro_unscope.hpp
@@ -14,6 +14,7 @@
 #undef NLOHMANN_BASIC_JSON_TPL
 #undef JSON_EXPLICIT
 #undef NLOHMANN_CAN_CALL_STD_FUNC_IMPL
+#undef JSON_INLINE_VARIABLE
 #undef JSON_NO_UNIQUE_ADDRESS
 
 #ifndef JSON_TEST_KEEP_MACROS
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index ebd4371f53..39ed7e95f8 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -2350,6 +2350,12 @@ using is_detected_convertible =
     #endif
 #endif
 
+#ifdef JSON_HAS_CPP_17
+    #define JSON_INLINE_VARIABLE inline
+#else
+    #define JSON_INLINE_VARIABLE
+#endif
+
 #if JSON_HEDLEY_HAS_ATTRIBUTE(no_unique_address)
     #define JSON_NO_UNIQUE_ADDRESS [[no_unique_address]]
 #else
@@ -4183,7 +4189,7 @@ namespace nlohmann
 namespace detail
 {
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
+inline void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_null()))
     {
@@ -4230,7 +4236,7 @@ void get_arithmetic_value(const BasicJsonType& j, ArithmeticType& val)
 }
 
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, typename BasicJsonType::boolean_t& b)
+inline void from_json(const BasicJsonType& j, typename BasicJsonType::boolean_t& b)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_boolean()))
     {
@@ -4240,7 +4246,7 @@ void from_json(const BasicJsonType& j, typename BasicJsonType::boolean_t& b)
 }
 
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, typename BasicJsonType::string_t& s)
+inline void from_json(const BasicJsonType& j, typename BasicJsonType::string_t& s)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_string()))
     {
@@ -4255,7 +4261,7 @@ template <
         std::is_assignable<StringType&, const typename BasicJsonType::string_t>::value
         && !std::is_same<typename BasicJsonType::string_t, StringType>::value
         && !is_json_ref<StringType>::value, int > = 0 >
-void from_json(const BasicJsonType& j, StringType& s)
+inline void from_json(const BasicJsonType& j, StringType& s)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_string()))
     {
@@ -4266,26 +4272,26 @@ void from_json(const BasicJsonType& j, StringType& s)
 }
 
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, typename BasicJsonType::number_float_t& val)
+inline void from_json(const BasicJsonType& j, typename BasicJsonType::number_float_t& val)
 {
     get_arithmetic_value(j, val);
 }
 
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, typename BasicJsonType::number_unsigned_t& val)
+inline void from_json(const BasicJsonType& j, typename BasicJsonType::number_unsigned_t& val)
 {
     get_arithmetic_value(j, val);
 }
 
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, typename BasicJsonType::number_integer_t& val)
+inline void from_json(const BasicJsonType& j, typename BasicJsonType::number_integer_t& val)
 {
     get_arithmetic_value(j, val);
 }
 
 template<typename BasicJsonType, typename EnumType,
          enable_if_t<std::is_enum<EnumType>::value, int> = 0>
-void from_json(const BasicJsonType& j, EnumType& e)
+inline void from_json(const BasicJsonType& j, EnumType& e)
 {
     typename std::underlying_type<EnumType>::type val;
     get_arithmetic_value(j, val);
@@ -4295,7 +4301,7 @@ void from_json(const BasicJsonType& j, EnumType& e)
 // forward_list doesn't have an insert method
 template<typename BasicJsonType, typename T, typename Allocator,
          enable_if_t<is_getable<BasicJsonType, T>::value, int> = 0>
-void from_json(const BasicJsonType& j, std::forward_list<T, Allocator>& l)
+inline void from_json(const BasicJsonType& j, std::forward_list<T, Allocator>& l)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
@@ -4312,7 +4318,7 @@ void from_json(const BasicJsonType& j, std::forward_list<T, Allocator>& l)
 // valarray doesn't have an insert method
 template<typename BasicJsonType, typename T,
          enable_if_t<is_getable<BasicJsonType, T>::value, int> = 0>
-void from_json(const BasicJsonType& j, std::valarray<T>& l)
+inline void from_json(const BasicJsonType& j, std::valarray<T>& l)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
@@ -4337,7 +4343,7 @@ auto from_json(const BasicJsonType& j, T (&arr)[N])  // NOLINT(cppcoreguidelines
 }
 
 template<typename BasicJsonType>
-void from_json_array_impl(const BasicJsonType& j, typename BasicJsonType::array_t& arr, priority_tag<3> /*unused*/)
+inline void from_json_array_impl(const BasicJsonType& j, typename BasicJsonType::array_t& arr, priority_tag<3> /*unused*/)
 {
     arr = *j.template get_ptr<const typename BasicJsonType::array_t*>();
 }
@@ -4381,8 +4387,8 @@ template<typename BasicJsonType, typename ConstructibleArrayType,
          enable_if_t<
              std::is_assignable<ConstructibleArrayType&, ConstructibleArrayType>::value,
              int> = 0>
-void from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr,
-                          priority_tag<0> /*unused*/)
+inline void from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr,
+                                 priority_tag<0> /*unused*/)
 {
     using std::end;
 
@@ -4439,7 +4445,7 @@ auto from_json(BasicJsonType&& j, identity_tag<std::array<T, N>> tag)
 }
 
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, typename BasicJsonType::binary_t& bin)
+inline void from_json(const BasicJsonType& j, typename BasicJsonType::binary_t& bin)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_binary()))
     {
@@ -4451,7 +4457,7 @@ void from_json(const BasicJsonType& j, typename BasicJsonType::binary_t& bin)
 
 template<typename BasicJsonType, typename ConstructibleObjectType,
          enable_if_t<is_constructible_object_type<BasicJsonType, ConstructibleObjectType>::value, int> = 0>
-void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
+inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_object()))
     {
@@ -4483,7 +4489,7 @@ template < typename BasicJsonType, typename ArithmeticType,
                !std::is_same<ArithmeticType, typename BasicJsonType::number_float_t>::value&&
                !std::is_same<ArithmeticType, typename BasicJsonType::boolean_t>::value,
                int > = 0 >
-void from_json(const BasicJsonType& j, ArithmeticType& val)
+inline void from_json(const BasicJsonType& j, ArithmeticType& val)
 {
     switch (static_cast<value_t>(j))
     {
@@ -4533,7 +4539,7 @@ std::pair<A1, A2> from_json_tuple_impl(BasicJsonType&& j, identity_tag<std::pair
 }
 
 template<typename BasicJsonType, typename A1, typename A2>
-void from_json_tuple_impl(BasicJsonType&& j, std::pair<A1, A2>& p, priority_tag<1> /*unused*/)
+inline void from_json_tuple_impl(BasicJsonType&& j, std::pair<A1, A2>& p, priority_tag<1> /*unused*/)
 {
     p = from_json_tuple_impl(std::forward<BasicJsonType>(j), identity_tag<std::pair<A1, A2>> {}, priority_tag<0> {});
 }
@@ -4545,7 +4551,7 @@ std::tuple<Args...> from_json_tuple_impl(BasicJsonType&& j, identity_tag<std::tu
 }
 
 template<typename BasicJsonType, typename... Args>
-void from_json_tuple_impl(BasicJsonType&& j, std::tuple<Args...>& t, priority_tag<3> /*unused*/)
+inline void from_json_tuple_impl(BasicJsonType&& j, std::tuple<Args...>& t, priority_tag<3> /*unused*/)
 {
     t = from_json_tuple_impl_base<BasicJsonType, Args...>(std::forward<BasicJsonType>(j), index_sequence_for<Args...> {});
 }
@@ -4565,7 +4571,7 @@ auto from_json(BasicJsonType&& j, TupleRelated&& t)
 template < typename BasicJsonType, typename Key, typename Value, typename Compare, typename Allocator,
            typename = enable_if_t < !std::is_constructible <
                                         typename BasicJsonType::string_t, Key >::value >>
-void from_json(const BasicJsonType& j, std::map<Key, Value, Compare, Allocator>& m)
+inline void from_json(const BasicJsonType& j, std::map<Key, Value, Compare, Allocator>& m)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
@@ -4585,7 +4591,7 @@ void from_json(const BasicJsonType& j, std::map<Key, Value, Compare, Allocator>&
 template < typename BasicJsonType, typename Key, typename Value, typename Hash, typename KeyEqual, typename Allocator,
            typename = enable_if_t < !std::is_constructible <
                                         typename BasicJsonType::string_t, Key >::value >>
-void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyEqual, Allocator>& m)
+inline void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyEqual, Allocator>& m)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
@@ -4604,7 +4610,7 @@ void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyE
 
 #if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
 template<typename BasicJsonType>
-void from_json(const BasicJsonType& j, std_fs::path& p)
+inline void from_json(const BasicJsonType& j, std_fs::path& p)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_string()))
     {
@@ -4626,13 +4632,18 @@ struct from_json_fn
 };
 }  // namespace detail
 
+#ifndef JSON_HAS_CPP_17
 /// namespace to hold default `from_json` function
 /// to see why this is required:
 /// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4381.html
 namespace // NOLINT(cert-dcl59-cpp,fuchsia-header-anon-namespaces,google-build-namespaces)
 {
-constexpr const auto& from_json = detail::static_const<detail::from_json_fn>::value; // NOLINT(misc-definitions-in-headers)
+#endif
+JSON_INLINE_VARIABLE constexpr const auto& from_json = // NOLINT(misc-definitions-in-headers)
+    detail::static_const<detail::from_json_fn>::value;
+#ifndef JSON_HAS_CPP_17
 } // namespace
+#endif
 } // namespace nlohmann
 
 // #include <nlohmann/detail/conversions/to_json.hpp>
@@ -5142,55 +5153,55 @@ struct external_constructor<value_t::object>
 
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_same<T, typename BasicJsonType::boolean_t>::value, int> = 0>
-void to_json(BasicJsonType& j, T b) noexcept
+inline void to_json(BasicJsonType& j, T b) noexcept
 {
     external_constructor<value_t::boolean>::construct(j, b);
 }
 
 template<typename BasicJsonType, typename CompatibleString,
          enable_if_t<std::is_constructible<typename BasicJsonType::string_t, CompatibleString>::value, int> = 0>
-void to_json(BasicJsonType& j, const CompatibleString& s)
+inline void to_json(BasicJsonType& j, const CompatibleString& s)
 {
     external_constructor<value_t::string>::construct(j, s);
 }
 
 template<typename BasicJsonType>
-void to_json(BasicJsonType& j, typename BasicJsonType::string_t&& s)
+inline void to_json(BasicJsonType& j, typename BasicJsonType::string_t&& s)
 {
     external_constructor<value_t::string>::construct(j, std::move(s));
 }
 
 template<typename BasicJsonType, typename FloatType,
          enable_if_t<std::is_floating_point<FloatType>::value, int> = 0>
-void to_json(BasicJsonType& j, FloatType val) noexcept
+inline void to_json(BasicJsonType& j, FloatType val) noexcept
 {
     external_constructor<value_t::number_float>::construct(j, static_cast<typename BasicJsonType::number_float_t>(val));
 }
 
 template<typename BasicJsonType, typename CompatibleNumberUnsignedType,
          enable_if_t<is_compatible_integer_type<typename BasicJsonType::number_unsigned_t, CompatibleNumberUnsignedType>::value, int> = 0>
-void to_json(BasicJsonType& j, CompatibleNumberUnsignedType val) noexcept
+inline void to_json(BasicJsonType& j, CompatibleNumberUnsignedType val) noexcept
 {
     external_constructor<value_t::number_unsigned>::construct(j, static_cast<typename BasicJsonType::number_unsigned_t>(val));
 }
 
 template<typename BasicJsonType, typename CompatibleNumberIntegerType,
          enable_if_t<is_compatible_integer_type<typename BasicJsonType::number_integer_t, CompatibleNumberIntegerType>::value, int> = 0>
-void to_json(BasicJsonType& j, CompatibleNumberIntegerType val) noexcept
+inline void to_json(BasicJsonType& j, CompatibleNumberIntegerType val) noexcept
 {
     external_constructor<value_t::number_integer>::construct(j, static_cast<typename BasicJsonType::number_integer_t>(val));
 }
 
 template<typename BasicJsonType, typename EnumType,
          enable_if_t<std::is_enum<EnumType>::value, int> = 0>
-void to_json(BasicJsonType& j, EnumType e) noexcept
+inline void to_json(BasicJsonType& j, EnumType e) noexcept
 {
     using underlying_type = typename std::underlying_type<EnumType>::type;
     external_constructor<value_t::number_integer>::construct(j, static_cast<underlying_type>(e));
 }
 
 template<typename BasicJsonType>
-void to_json(BasicJsonType& j, const std::vector<bool>& e)
+inline void to_json(BasicJsonType& j, const std::vector<bool>& e)
 {
     external_constructor<value_t::array>::construct(j, e);
 }
@@ -5203,39 +5214,39 @@ template < typename BasicJsonType, typename CompatibleArrayType,
                          !std::is_same<typename BasicJsonType::binary_t, CompatibleArrayType>::value&&
                          !is_basic_json<CompatibleArrayType>::value,
                          int > = 0 >
-void to_json(BasicJsonType& j, const CompatibleArrayType& arr)
+inline void to_json(BasicJsonType& j, const CompatibleArrayType& arr)
 {
     external_constructor<value_t::array>::construct(j, arr);
 }
 
 template<typename BasicJsonType>
-void to_json(BasicJsonType& j, const typename BasicJsonType::binary_t& bin)
+inline void to_json(BasicJsonType& j, const typename BasicJsonType::binary_t& bin)
 {
     external_constructor<value_t::binary>::construct(j, bin);
 }
 
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_convertible<T, BasicJsonType>::value, int> = 0>
-void to_json(BasicJsonType& j, const std::valarray<T>& arr)
+inline void to_json(BasicJsonType& j, const std::valarray<T>& arr)
 {
     external_constructor<value_t::array>::construct(j, std::move(arr));
 }
 
 template<typename BasicJsonType>
-void to_json(BasicJsonType& j, typename BasicJsonType::array_t&& arr)
+inline void to_json(BasicJsonType& j, typename BasicJsonType::array_t&& arr)
 {
     external_constructor<value_t::array>::construct(j, std::move(arr));
 }
 
 template < typename BasicJsonType, typename CompatibleObjectType,
            enable_if_t < is_compatible_object_type<BasicJsonType, CompatibleObjectType>::value&& !is_basic_json<CompatibleObjectType>::value, int > = 0 >
-void to_json(BasicJsonType& j, const CompatibleObjectType& obj)
+inline void to_json(BasicJsonType& j, const CompatibleObjectType& obj)
 {
     external_constructor<value_t::object>::construct(j, obj);
 }
 
 template<typename BasicJsonType>
-void to_json(BasicJsonType& j, typename BasicJsonType::object_t&& obj)
+inline void to_json(BasicJsonType& j, typename BasicJsonType::object_t&& obj)
 {
     external_constructor<value_t::object>::construct(j, std::move(obj));
 }
@@ -5245,13 +5256,13 @@ template <
     enable_if_t < !std::is_constructible<typename BasicJsonType::string_t,
                   const T(&)[N]>::value, // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
                   int > = 0 >
-void to_json(BasicJsonType& j, const T(&arr)[N]) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
+inline void to_json(BasicJsonType& j, const T(&arr)[N]) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
 {
     external_constructor<value_t::array>::construct(j, arr);
 }
 
 template < typename BasicJsonType, typename T1, typename T2, enable_if_t < std::is_constructible<BasicJsonType, T1>::value&& std::is_constructible<BasicJsonType, T2>::value, int > = 0 >
-void to_json(BasicJsonType& j, const std::pair<T1, T2>& p)
+inline void to_json(BasicJsonType& j, const std::pair<T1, T2>& p)
 {
     j = { p.first, p.second };
 }
@@ -5259,26 +5270,26 @@ void to_json(BasicJsonType& j, const std::pair<T1, T2>& p)
 // for https://github.com/nlohmann/json/pull/1134
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_same<T, iteration_proxy_value<typename BasicJsonType::iterator>>::value, int> = 0>
-void to_json(BasicJsonType& j, const T& b)
+inline void to_json(BasicJsonType& j, const T& b)
 {
     j = { {b.key(), b.value()} };
 }
 
 template<typename BasicJsonType, typename Tuple, std::size_t... Idx>
-void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<Idx...> /*unused*/)
+inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<Idx...> /*unused*/)
 {
     j = { std::get<Idx>(t)... };
 }
 
 template<typename BasicJsonType, typename T, enable_if_t<is_constructible_tuple<BasicJsonType, T>::value, int > = 0>
-void to_json(BasicJsonType& j, const T& t)
+inline void to_json(BasicJsonType& j, const T& t)
 {
     to_json_tuple_impl(j, t, make_index_sequence<std::tuple_size<T>::value> {});
 }
 
 #if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
 template<typename BasicJsonType>
-void to_json(BasicJsonType& j, const std_fs::path& p)
+inline void to_json(BasicJsonType& j, const std_fs::path& p)
 {
     j = p.string();
 }
@@ -5295,13 +5306,18 @@ struct to_json_fn
 };
 }  // namespace detail
 
+#ifndef JSON_HAS_CPP_17
 /// namespace to hold default `to_json` function
 /// to see why this is required:
 /// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4381.html
 namespace // NOLINT(cert-dcl59-cpp,fuchsia-header-anon-namespaces,google-build-namespaces)
 {
-constexpr const auto& to_json = detail::static_const<detail::to_json_fn>::value; // NOLINT(misc-definitions-in-headers)
+#endif
+JSON_INLINE_VARIABLE constexpr const auto& to_json = // NOLINT(misc-definitions-in-headers)
+    detail::static_const<detail::to_json_fn>::value;
+#ifndef JSON_HAS_CPP_17
 } // namespace
+#endif
 } // namespace nlohmann
 
 // #include <nlohmann/detail/meta/identity_tag.hpp>
@@ -23430,6 +23446,7 @@ inline nlohmann::json::json_pointer operator "" _json_pointer(const char* s, std
 #undef NLOHMANN_BASIC_JSON_TPL
 #undef JSON_EXPLICIT
 #undef NLOHMANN_CAN_CALL_STD_FUNC_IMPL
+#undef JSON_INLINE_VARIABLE
 #undef JSON_NO_UNIQUE_ADDRESS
 
 #ifndef JSON_TEST_KEEP_MACROS