On Thu, Mar 12, 2015 at 7:55 PM, Hans Hagen <pragma@wxs.nl> wrote:

> it's actually a bug ... it is ok to map an invalid character in the input
> to 0xFFFD, halt and continue when permitted, but the method used in luatex
> thereby obscures a valid 0xFFFD in the input
>
>  FFFD  REPLACEMENT CHARACTER
• used to replace an incoming character whose
value is unknown or unrepresentable in
Unicode

The meaning of FFFD is not "typeset a question mark on a black box" as in �
(which depends to font in anycase so in principle it's possible to see
something completely different in a new version of the font)
but to signal  something potentially wrong with a symbol that currently in
most cases is �.
Misusing the meaning  is not  bad di per se, but in this specific case
I think luatex is correct to be conservative and ask to the user what to do;
context --batchmode
typesets the document,
writes the messages on the log,
and ends with -1 , so an automatic agent is also alerted.




-- 
luigi