add a standard function to write a replacement character

ostd/format.hh

@@ -988,14 +988,6 @@ private:
         for (int w = p_width - int(n); --w >= 0; writer.put(c));
     }
 
-    template<typename R>
-    void write_replacement(R &writer) const {
-        /* Unicode replacement character */
-        writer.put(0xEF);
-        writer.put(0xBF);
-        writer.put(0xBD);
-    }
-
     template<typename R>
     void write_char_raw(R &writer, char val) const {
         writer.put(val);
@@ -1009,7 +1001,7 @@ private:
     template<typename R>
     void write_char_raw(R &writer, char32_t val) const {
         if (!utf::encode<char>(writer, val)) {
-            write_replacement(writer);
+            utf::replace<char>(writer);
         }
     }
 
@@ -1048,7 +1040,7 @@ private:
                     }
                     val.pop_front();
                 } else if (!utf::encode<char>(writer, val)) {
-                    write_replacement(writer);
+                    utf::replace<char>(writer);
                     val.pop_front();
                 }
             }

ostd/string.hh

@@ -851,6 +851,24 @@ namespace utf {
         return encode<unicode_t<N>>(sink, r);
     }
 
+    template<typename C, typename R>
+    inline std::size_t replace(R &sink) {
+        if constexpr(max_units<C> > 2) {
+            sink.put(0xEF);
+            sink.put(0xBF);
+            sink.put(0xBD);
+            return 3;
+        } else {
+            sink.put(0xFFFD);
+        }
+        return 1;
+    }
+
+    template<std::size_t N, typename R>
+    inline std::size_t replace(R &sink) {
+        return replace<unicode_t<N>>(sink);
+    }
+
     /* @brief Get the number of Unicode code points in a string.
      *
      * This function keeps reading Unicode code points while it can and