bidi properties from uniprop tables

["Stephen J. Turnbull" <stephen@xemacs.org>]

Eli Zaretskii writes:

 > Such undefined characters should not normally appear in any text,

Somebody misread the standard, I think.  In Unicode, everything in the
space 0 -- (2^20-1) is a character, with a few well-defined exceptions
called non-characters.  Most, however do not (yet) appear in the UCD,
but are reserved for future standardization.  Since the standard
regularly adds new characters, older versions of Emacs are very likely
to encounter reserved characters "normally" in "some" texts.

The UCD makes provision for this by providing definitions of some
properties for *all code points* (not merely all defined characters).
Unassigned (including noncharacter) code points automatically get the
General_Category property with value Cn (Unicode 6.0, section 4.5,
Table 4.9).  They also automatically get the Name property, with value
"" (the null string, Unicode 6.0, section 4.8, "Formal Definition of
the Name Property", p. 132.

 > I made the code in bidi.c defensive about what it gets from the

Maybe that should be an assert, since a null return is an Emacs bug.

 > uniprop table, but the question is, should we do something to never
 > have nil in Lisp or zero in C return from these APIs?

Yes, a non-nil property list is required by the standard for all code
points (not merely "all characters"), and it is obvious that in this
case conforming to the standard is useful.