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

--- src/common/json.hpp.orig	2018-04-09 20:44:59 UTC
+++ src/common/json.hpp
@@ -840,6 +840,16 @@ 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
 {
@@ -850,6 +860,15 @@ 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
 {
@@ -1132,6 +1151,25 @@ 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*>();
 }