bug#24672: 26.0.50; viper-mode + c-mode: "ce" fails in cpp-continued stmt

bug#24672: 26.0.50; viper-mode + c-mode: "ce" fails in cpp-continued stmt

[Jim Meyering]

This is with an emacs binary I built from "master" yesterday.

Reproduce with this:

  printf '%s\n' '#define abcde \\' x > k.c; emacs -q -f viper-mode k.c

Then, hit "n" at the viperize prompt and you'll see this
on the first line:

  #define abcde \

Hit "W" to move the cursor to the "a". Then type "ce(" to
change the 5-byte "abcde" to a "(".  I was surprised to see
the "abcde" still there, and an additional backslash appended:

  #define (abcde \                                \

I expected to see this:

  #define ( \

In GNU Emacs 26.0.50.1 (x86_64-unknown-linux-gnu)
 of 2016-10-10 built on hx.meyering.net
 Repository revision: 8b1be820ce02379c9ebd124703e04eee65779662

Configured using:
 'configure --prefix=/p/p/emacs-2016-10-10.08h35 --without-gpm
  --without-x --with-x-toolkit=no --with-png=no --with-jpeg=no
   --with-sound=no'

bug#24672: writing a test for Bug#24672 (viper mode malfunction while inserting a paren in continued cpp directive)

[Jim Meyering]

This is a corner case.
When I start in viper mode with this file contents:

  #define z_b\

and then change the "z" to a parenthesis (either one), I get this:

  #define (z_b^I^I^I^I^I\$

[I have represented each TAB as "^I", of course]
Rather surprising to see all of those TAB characters inserted. To
reproduce, run this:

  mkdir ~/empty && printf '#define z_b\\\n' > k.c \
    && HOME=$HOME/empty /bin/emacs -Q -f viper-mode k.c

hit "n", "n", and "5" at the successive prompts, then e.g., "fzs(" to
perform the change and to see the surprising result.

I tried to write a test case to encapsulate the above, but so far have
failed, because when run via the test, viper-mode does what one would
expect.

commit 3309c37d8c42b2fd002b0d965dafc4be3b6d3e44
Author: Jim Meyering <meyering@fb.com>
Date:   Wed Oct 12 08:57:48 2016 -0700

    viper-tests.el: add a test for bug #24672

diff --git a/test/lisp/emulation/viper-tests.el
b/test/lisp/emulation/viper-tests.el
index 2c63b24..85d83aa 100644
--- a/test/lisp/emulation/viper-tests.el
+++ b/test/lisp/emulation/viper-tests.el
@@ -99,6 +99,19 @@ viper-test-undo-kmacro
       ]
      ))))

+(ert-deftest viper-test-insert-paren-on-cpp-continued-line()
+  "Test for bug #24672:
+Insert '#define z_b\', then change the 'z' to a parenthesis (open or closed)"
+  (should
+   (equal
+    "#define (_b\\\n"
+    (viper-test-undo-kmacro
+     [
+      ?i ?# ?d ?e ?f ?i ?n ?e ?  ?z ?_ ?b ?\\ escape
+         ?F ?z ?s ?\( escape
+         ])
+    )))
+
 (ert-deftest viper-test-undo-2 ()
   "Test for VI like undo behavior.

Is there some test set-up I can perform to make that test work like
what I outlined above?