src/Pure/General/utf8.scala

--- a/src/Pure/General/utf8.scala	Sat Jan 09 22:41:08 2021 +0100
+++ b/src/Pure/General/utf8.scala	Sun Jan 10 13:04:29 2021 +0100
@@ -84,7 +84,7 @@
   def decode_chars(decode: String => String,
     buffer: Array[Byte], start: Int, end: Int): CharSequence =
   {
-    require(0 <= start && start <= end && end <= buffer.length)
+    require(0 <= start && start <= end && end <= buffer.length, "bad decode range")
     new Decode_Chars(decode, buffer, start, end)
   }
 }