Inline function definitions for isdigit and isxdigit?

[Florian Weimer <fweimer at redhat dot com>]

Can we provide inline definitions for isdigit and xdigit?

POSIX says (for the digit character class):

“
In a locale definition file, only the digits <zero>, <one>, <two>, 
<three>, <four>, <five>, <six>, <seven>, <eight>, and <nine> shall be 
specified, and in contiguous ascending sequence by numerical value. The 
digits <zero> to <nine> of the portable character set are automatically 
included in this class.
”

This means it's fixed to '0' .. '9' for our purposes (our locales must 
be ASCII-transparent at least as far as the digits are concerned).

For xdigit, one can have more than two sequences of 'A' .. 'F' letters:

“
In a locale definition file, only the characters defined for the class 
digit shall be specified, in contiguous ascending sequence by numerical 
value, followed by one or more sets of six characters representing the 
hexadecimal digits 10 to 15 inclusive, with each set in ascending order 
(for example, <A>, <B>, <C>, <D>, <E>, <F>, <a>, <b>, <c>, <d>, <e>, <f>).
”

But I wonder how useful this is in practice.  One might be tempted to 
define FULLWIDTH LATIN SMALL LETTER A (U+FF41) as a hexadecimal digit. 
But this does not make much sense because FULLWIDTH DIGIT ZERO (U+FF10) 
and the other double-width digits cannot be defined as hexadecimal 
digits.  I wonder if all reasonable extension candidates for xdigit are 
similar.

Even for isxdigit, an inline definition would result in a shorter 
machine code sequence than the current implementation (maybe not if the 
result of the call to __ctype_b_loc can be reused).  For isdigit, the 
case is even clearer, even with a shared __ctype_b_loc value.

Thanks,
Florian