This simple C program fails to compile when using glibc 2.8 (I'm using gcc 4.3.0
on Fedora 9).

#include <wchar.h>
#include <wctype.h>
#include <stddef.h>
ptrdiff_t i;

The error is:

foo.c:4: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘i’

The error is because the typedef for ptrdiff_t was not defined by <stddef.h>. 
That happens because wctype.h does this:

/* Get wint_t from <wchar.h>.  */
# define __need_wint_t
# include <wchar.h>

Unfortunately, if the program has already #included <wchar.h>, then _WCHAR_H is
defined and <wchar.h> does nothing, including not doing a #undef of
__need_wint_t.  The presence of the __need_wint_t then causes <stddef.h> to only
define wint_t, and not define the types it should normally define when it is
included directly.

It seems to me that either wctype.h should not #include wchar.h, or wchar.h
should reliably #undef __need_wint_t.