Fix iterator_input_adapter for types not supported by char_traits

include/nlohmann/detail/input/input_adapters.hpp

@@ -132,10 +132,70 @@ class input_stream_adapter
 };
 #endif  // JSON_NO_IO
 
+template<typename T>
+struct has_char_traits : std::false_type {};
+
+template<>
+struct has_char_traits<char> : std::true_type {};
+
+template<>
+struct has_char_traits<wchar_t> : std::true_type {};
+
+#ifdef JSON_HAS_CPP_20
+template<>
+struct has_char_traits<char8_t> : std::true_type {};
+
+template<>
+struct has_char_traits<char16_t> : std::true_type {};
+
+template<>
+struct has_char_traits<char32_t> : std::true_type {};
+#endif
+
+template<typename T>
+struct is_iterator_of_char : has_char_traits<typename std::iterator_traits<T>::value_type> {};
+
 // General-purpose iterator-based adapter. It might not be as fast as
 // theoretically possible for some containers, but it is extremely versatile.
-template<typename IteratorType>
+template<typename IteratorType, typename = void>
 class iterator_input_adapter
+{
+  public:
+    using char_type = char;
+    using value_type = typename std::iterator_traits<IteratorType>::value_type;
+
+    iterator_input_adapter(IteratorType first, IteratorType last)
+        : current(std::move(first)), end(std::move(last))
+    {}
+
+    typename std::char_traits<char>::int_type get_character()
+    {
+        if (JSON_HEDLEY_LIKELY(current != end))
+        {
+            // Cast to unsigned to avoid EOF and 0xFF having same value
+            auto result = static_cast<typename std::make_unsigned<value_type>::type>(*current);
+            std::advance(current, 1);
+            return result;
+        }
+
+        return std::char_traits<char>::eof();
+    }
+
+  private:
+    IteratorType current;
+    IteratorType end;
+
+    template<typename BaseInputAdapter, size_t T>
+    friend struct wide_string_input_helper;
+
+    bool empty() const
+    {
+        return current == end;
+    }
+};
+
+template<typename IteratorType>
+class iterator_input_adapter<IteratorType, enable_if_t<is_iterator_of_char<IteratorType>::value>>
 {
   public:
     using char_type = typename std::iterator_traits<IteratorType>::value_type;

single_include/nlohmann/json.hpp

@@ -6208,10 +6208,70 @@ class input_stream_adapter
 };
 #endif  // JSON_NO_IO
 
+template<typename T>
+struct has_char_traits : std::false_type {};
+
+template<>
+struct has_char_traits<char> : std::true_type {};
+
+template<>
+struct has_char_traits<wchar_t> : std::true_type {};
+
+#ifdef JSON_HAS_CPP_20
+template<>
+struct has_char_traits<char8_t> : std::true_type {};
+
+template<>
+struct has_char_traits<char16_t> : std::true_type {};
+
+template<>
+struct has_char_traits<char32_t> : std::true_type {};
+#endif
+
+template<typename T>
+struct is_iterator_of_char : has_char_traits<typename std::iterator_traits<T>::value_type> {};
+
 // General-purpose iterator-based adapter. It might not be as fast as
 // theoretically possible for some containers, but it is extremely versatile.
-template<typename IteratorType>
+template<typename IteratorType, typename Enable = void>
 class iterator_input_adapter
+{
+  public:
+    using char_type = char;
+    using value_type = typename std::iterator_traits<IteratorType>::value_type;
+
+    iterator_input_adapter(IteratorType first, IteratorType last)
+        : current(std::move(first)), end(std::move(last))
+    {}
+
+    typename std::char_traits<char>::int_type get_character()
+    {
+        if (JSON_HEDLEY_LIKELY(current != end))
+        {
+            // Cast to unsigned to avoid EOF and 0xFF having same value
+            auto result = static_cast<typename std::make_unsigned<value_type>::type>(*current);
+            std::advance(current, 1);
+            return result;
+        }
+
+        return std::char_traits<char>::eof();
+    }
+
+  private:
+    IteratorType current;
+    IteratorType end;
+
+    template<typename BaseInputAdapter, size_t T>
+    friend struct wide_string_input_helper;
+
+    bool empty() const
+    {
+        return current == end;
+    }
+};
+
+template<typename IteratorType>
+class iterator_input_adapter<IteratorType, enable_if_t<is_iterator_of_char<IteratorType>::value>>
 {
   public:
     using char_type = typename std::iterator_traits<IteratorType>::value_type;