bug#35254: 27.0.50; cc-mode/electric-pair-mode/electric-layout-mode: bad trailing whitespace behavior in cc-mode

[Basil L. Contovounesios]

Alan Mackenzie <acm@muc.de> writes:

> On Sat, May 11, 2019 at 20:34:51 +0100, Basil L. Contovounesios wrote:
>> Alan Mackenzie <acm@muc.de> writes:
>
>> > On Sat, May 11, 2019 at 10:06:42 -0400, Noam Postavsky wrote:
>> >> Alan Mackenzie <acm@muc.de> writes:
>
>> >> Of course c-mode could rebind it in its mode map (I considered making
>> >> `electric-newline-and-maybe-indent' consult `electric-indent-functions'
>> >> as well but that won't work because that hook is supposed to run after
>> >> the character is inserted).
>
>> > I think we've got enough foggy complexity in the area as it is.  I
>> > suppose CC Mode could bind <CR> to `newline-and-indent', but there's now
>> > no longer a clean function which does what `newline' used to do, to bind
>> > onto C-j.
>
>> Sorry if my question is completely naive or irrelevant (I haven't read
>> the discussion very carefully), but how does the command
>> c-context-line-break, which is described under "Making the <RET> key
>> indent the new line" in (info "(ccmode) Getting Started") relate to this
>> issue, if at all?
>
> c-context-line-break doesn't really have much to say in the matter.  The
> function is mainly about how to indent the _new_ line, and inserting
> various continuation markers.
>
> This bug is about trailing space in the _old_ line not getting removed
> on typing <CR>, about which c-context-line-break has nothing to say.

AFAICS c-context-line-break removes trailing space on the old line:

0. emacs -Q
1. C-x h C-w
2. M-x c-mode RET
3. int main() {
4. RET RET

Line 2 now contains two trailing spaces.

5. M-x c-context-line-break RET

Line 3 is now empty (has no trailing space).

Have I misunderstood something?

Thanks,

-- 
Basil