https://github.com/nlohmann/json/commit/8165707990e4

--- 3rd-party/json.hpp.orig	2018-06-24 19:26:15 UTC

+++ 3rd-party/json.hpp

@@ -353,6 +353,16 @@ struct is_compatible_object_type_impl<true, RealType, 

         std::is_constructible<typename RealType::mapped_type, typename CompatibleObjectType::mapped_type>::value;

 };

+template<bool B, class RealType, class CompatibleStringType>

+struct is_compatible_string_type_impl : std::false_type {};

+

+template<class RealType, class CompatibleStringType>

+struct is_compatible_string_type_impl<true, RealType, CompatibleStringType>

+{

+    static constexpr auto value =

+        std::is_same<typename RealType::value_type, typename CompatibleStringType::value_type>::value;

+};

+

 template<class BasicJsonType, class CompatibleObjectType>

 struct is_compatible_object_type

 {

@@ -363,6 +373,15 @@ struct is_compatible_object_type

                                   typename BasicJsonType::object_t, CompatibleObjectType >::value;

 };

+template<class BasicJsonType, class CompatibleStringType>

+struct is_compatible_string_type

+{

+    static auto constexpr value = is_compatible_string_type_impl <

+                                  conjunction<negation<std::is_same<void, CompatibleStringType>>,

+                                  has_value_type<CompatibleStringType>>::value,

+                                  typename BasicJsonType::string_t, CompatibleStringType >::value;

+};

+

 template<typename BasicJsonType, typename T>

 struct is_basic_json_nested_type

 {

@@ -974,6 +993,25 @@ void from_json(const BasicJsonType& j, typename BasicJ

     {

         JSON_THROW(type_error::create(302, "type must be string, but is " + std::string(j.type_name())));

     }

+    s = *j.template get_ptr<const typename BasicJsonType::string_t*>();

+}

+

+template <

+    typename BasicJsonType, typename CompatibleStringType,

+    enable_if_t <

+        is_compatible_string_type<BasicJsonType, CompatibleStringType>::value and

+        not std::is_same<typename BasicJsonType::string_t,

+                         CompatibleStringType>::value and

+        std::is_constructible <

+            BasicJsonType, typename CompatibleStringType::value_type >::value,

+        int > = 0 >

+void from_json(const BasicJsonType& j, CompatibleStringType& s)

+{

+    if (JSON_UNLIKELY(not j.is_string()))

+    {

+        JSON_THROW(type_error::create(302, "type must be string, but is " + std::string(j.type_name())));

+    }

+

     s = *j.template get_ptr<const typename BasicJsonType::string_t*>();

 }