sink-writing decode

ostd/string.hh

@@ -787,6 +787,15 @@ namespace utf {
      */
     bool decode(wstring_range &r, char32_t &ret) noexcept;
 
+    template<typename R, typename C>
+    bool decode(R &sink, basic_char_range<C const> &r) {
+        if (char32_t ch; utf::decode(r, ch)) {
+            sink.put(ch);
+            return true;
+        }
+        return false;
+    }
+
     namespace detail {
         std::size_t u8_encode(
             char (&ret)[4], char32_t ch