src/java.base/share/classes/sun/nio/cs/UTF_8.java

@@ -547,10 +547,14 @@
                     da[dp++] = replacement().charAt(0);
                 }
             }
             return dp;
         }
+
+        public boolean isASCIICompatible() {
+            return true;
+        }
     }
 
     private static final class Encoder extends CharsetEncoder
                                  implements ArrayEncoder {
 

@@ -740,7 +744,11 @@
                     da[dp++] = (byte)(0x80 | (c & 0x3f));
                 }
             }
             return dp;
         }
+
+        public boolean isASCIICompatible() {
+            return true;
+        }
     }
 }