'cpoptions': "_" flag to toggle `cw` behaviour #6235

`cw` and `cW` behave like `ce` and `cE` respectively. This is
inconsistent compared to `dw` and `dW`.
Introduce a new cpoptions flag "_" to toggle the Vi behavior.

Closes #6234
Patch-by: Christian Brabandt <cblists@256bit.org>

References:
https://github.com/chrisbra/vim-mq-patches/blob/master/cpo_changeword
https://groups.google.com/d/msg/vim_use/aaBqT6ECkA4/ALf4odKzEDgJ
https://groups.google.com/d/msg/vim_dev/Dpn3xtUF16I/T6JcOPKN6usJ
http://www.reddit.com/r/vim/comments/26nut8/why_does_cw_work_like_ce/

5 files changed, 13 insertions, 7 deletions

diff --git a/runtime/doc/motion.txt b/runtime/doc/motion.txt
index 606fd53fee..be3170cf92 100644
--- a/runtime/doc/motion.txt
+++ b/runtime/doc/motion.txt
@@ -401,8 +401,7 @@ a range of folded lines.  "b" and "B" move to the start of the first word or
 WORD before the fold.
 
 Special case: "cw" and "cW" are treated like "ce" and "cE" if the cursor is
-on a non-blank.  This is because "cw" is interpreted as change-word, and a
-word does not include the following white space.
+on a non-blank.  This is Vi-compatible, see |cpo-_| to change the behavior.
 
 Another special case: When using the "w" motion in combination with an
 operator and the last word moved over is at the end of a line, the end of
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 25dca5fb51..e28e4e59a8 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -1829,6 +1829,9 @@ A jump table for the options with a short description can be found at |Q_op|.
 			character, the cursor won't move. When not included,
 			the cursor would skip over it and jump to the
 			following occurrence.
+								*cpo-_*
+		_	When using |cw| on a word, do not include the
+			whitespace following the word in the motion.
 
 						*'cscopepathcomp'* *'cspc'*
 'cscopepathcomp' 'cspc'	number	(default 0)
diff --git a/runtime/doc/vim_diff.txt b/runtime/doc/vim_diff.txt
index bcc5d03ff0..bea69eb924 100644
--- a/runtime/doc/vim_diff.txt
+++ b/runtime/doc/vim_diff.txt
@@ -108,6 +108,7 @@ Some `CTRL-SHIFT-...` key chords are distinguished from `CTRL-...` variants
   <C-Tab>, <C-S-Tab>, <C-BS>, <C-S-BS>, <C-Enter>, <C-S-Enter>
 
 Options:
+  'cpoptions' flags: |cpo-_|
   'inccommand' shows interactive results for |:substitute|-like commands
   'statusline' supports unlimited alignment sections
   'tabline' %@Func@foo%X can call any function on mouse-click
diff --git a/src/nvim/normal.c b/src/nvim/normal.c
index 92b1cf2d55..d010b4b3a7 100644
--- a/src/nvim/normal.c
+++ b/src/nvim/normal.c
@@ -7193,8 +7193,10 @@ static void nv_wordcmd(cmdarg_T *cap)
       // Another strangeness: When standing on the end of a word "ce" will
       // change until the end of the next word, but "cw" will change only one
       // character!  This is done by setting "flag".
-      cap->oap->inclusive = true;
-      word_end = true;
+      if (vim_strchr(p_cpo, CPO_CHANGEW) != NULL) {
+        cap->oap->inclusive = true;
+        word_end = true;
+      }
       flag = true;
     }
   }
diff --git a/src/nvim/option_defs.h b/src/nvim/option_defs.h
index 9c6393e014..1fd6dc9c91 100644
--- a/src/nvim/option_defs.h
+++ b/src/nvim/option_defs.h
@@ -129,9 +129,10 @@
 #define CPO_REGAPPEND   '>'     /* insert NL when appending to a register */
 #define CPO_SCOLON      ';'     /* using "," and ";" will skip over char if
                                  * cursor would not move */
-/* default values for Vim and Vi */
-#define CPO_VIM         "aABceFs"
-#define CPO_VI          "aAbBcCdDeEfFiIJkKlLmMnoOpPqrRsStuvWxXyZ$!%+<>;"
+#define CPO_CHANGEW     '_'     // "cw" special-case
+// default values for Vim and Vi
+#define CPO_VIM         "aABceFs_"
+#define CPO_VI          "aAbBcCdDeEfFiIJkKlLmMnoOpPqrRsStuvWxXyZ$!%+<>;_"
 
 /* characters for p_ww option: */
 #define WW_ALL          "bshl<>[],~"