bug#8657: 24.0.50; Emacs code that seems to use `*-single-char-property-change' incorrectly

[Drew Adams]

`(next|previous)-single-char-property-change' behaves differently in a
couple of ways from `(next|previous)-single-property-change', besides
the difference of overlay vs text properties.
 
In particular, IIUC, the return value can never be nil.
Yet I see code such as this:
 
gnus-summary-show-thread:
(or (next-single-char-property-change end 'invisible) (point-max))
 
comint-next-prompt:
(setq pos (next-single-char-property-change pos 'field))
(cond (or (null pos)...
 
Isn't this incorrect?

In GNU Emacs 24.0.50.1 (i386-mingw-nt5.1.2600)
 of 2011-05-10 on 3249CTO
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.5) --no-opt --cflags
-Ic:/build/include'