...

common/adc_traits.h

@@ -67,6 +67,25 @@ concept adc_range_of_output_char_range =
 template <typename R>
 concept adc_range_of_view_or_output_char_range = adc_range_of_view_char_range<R> || adc_output_char_range<R>;
 
+
+template <adc_char_range R>
+struct adc_char_identity {
+    template <adc_char_range T>
+        requires std::same_as<R, T>
+    constexpr T&& operator()(T&& v)  // like std::identity
+    {
+        return std::forward<T>(v);
+    }
+
+    template <adc_char_range T>
+        requires(!std::same_as<R, T> &&
+                 std::is_constructible_v<R, std::ranges::iterator_t<T>, std::ranges::sentinel_t<T>>)
+    constexpr R operator()(T&& v)
+    {
+        return {v.begin(), v.end()};
+    }
+};
+
 // deduce returned type of callable
 // template <typename T>
 // using adc_retval_t = std::invoke_result_t<std::remove_cvref_t<T>>;