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

[Alan Mackenzie]

Hello, Basil.

On Sun, May 12, 2019 at 22:45:09 +0100, Basil L. Contovounesios wrote:
> Alan Mackenzie <acm@muc.de> writes:

> > On Sat, May 11, 2019 at 20:34:51 +0100, Basil L. Contovounesios wrote:

> >> 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?

Er, no.  You're right, c-context-line-break does indeed remove the
trailing WS, at least on normal code lines.  Sorry about the mistake.

But I don't think I've really understood how this observation fits in
with the bug scenario.  The bug is about the current master's default
binding of <CR> (namely newline) not removing the trailing whitespace
from the line it's typed in.

I think you might be suggesting binding c-context-line-break to <CR> in
CC Mode as a workaround for the problem; or possibly using its ideas to
code up a CC Mode version of newline.

I still think the bug should be fixed in the Emacs core, so that other
modes which want the old line to have trailing spaces removed, yet don't
use electric-indent-mode, will just work.

> Thanks,

> -- 
> Basil

-- 
Alan Mackenzie (Nuremberg, Germany).