WITH-CSTRING coerces Unicode strings.

src/CHANGELOG

@@ -18,6 +18,9 @@ ECL 9.5:
     #.(MAKE-PATHNAME :HOST NIL :DEVICE NIL :DIRECTORY '(:RELATIVE) :NAME NIL
                      :TYPE NIL :VERSION :NEWEST :DEFAULTS NIL)
 
+ - WITH-CSTRING automatically coerces extended strings to base strings,
+   signalling an error when this is not possible.
+
 * Bugs:
 
  - Remove an obsolete #if statement for Solaris that broke current builds

src/lsp/ffi.lsp

@@ -391,13 +391,13 @@
   ;; This enforces that the string contains only as many characters as the
   ;; fill-pointer determines Since ECL always sets a 0 character after the
   ;; last element of a string, this way, the string is always zero-terminated
-  `(copy-seq ,object))
+  `(si:copy-to-simple-base-string ,object))
 
 (defmacro free-cstring (object)
   object)
 
 (defmacro with-cstring ((cstring string) &body body)
-  `(let ((,cstring (copy-seq ,string))) ,@body))
+  `(let ((,cstring (convert-to-cstring ,string))) ,@body))
 
 (defmacro with-cstrings (bindings &rest body)
   (if bindings