In substring, only return arg string if it's in NFC.

diff --git a/src/runtime/ustring.scm b/src/runtime/ustring.scm
index bfbcfb55636bab4ff4a7c1988c2a3d620caab0b2..b84c3a5ab0fc3842f8cbc9d01e7cf6c5c18633df 100644 (file)
--- a/src/runtime/ustring.scm
+++ b/src/runtime/ustring.scm
@@ -387,7 +387,7 @@ USA.
     (if (and (fix:= start 0)
             (fix:= end len)
             (not (slice? string))
-            (not (ustring-mutable? string)))
+            (ustring-in-nfc? string))
        string
        (translate-slice string start end
          (lambda (string start end)