feat: ":write!" skips "file changed" warning #18665

Problem:
Cannot opt-out of "WARNING: The file has been changed since reading
it!!!", even with ":write!".

Solution:
Change ":write!" to skip the warning.

closes #7270

runtime/doc/editing.txt

@@ -941,7 +941,7 @@ WRITING WITH MULTIPLE BUFFERS				*buffer-write*
 
 
 Vim will warn you if you try to overwrite a file that has been changed
-elsewhere.  See |timestamp|.
+elsewhere (unless "!" was used).  See |timestamp|.
 
 			    *backup* *E207* *E506* *E507* *E508* *E509* *E510*
 If you write to an existing file (but do not append) while the 'backup',
@@ -1481,8 +1481,8 @@ doing something there and closing it should be OK (if there are no side
 effects from other autocommands).  Closing unrelated windows and buffers will
 get you into trouble.
 
-Before writing a file the timestamp is checked.  If it has changed, Vim will
-ask if you really want to overwrite the file:
+Before writing a file, the timestamp is checked (unless "!" was used).
+If it has changed, Vim will ask if you really want to overwrite the file:
 
 	WARNING: The file has been changed since reading it!!!
 	Do you really want to write to it (y/n)?

runtime/doc/vim_diff.txt

@@ -372,6 +372,7 @@ Lua interface (|lua.txt|):
 Commands:
   |:doautocmd| does not warn about "No matching autocommands".
   |:wincmd| accepts a count.
+  `:write!` does not show a prompt if the file was updated externally.
 
 Command line completion:
   The meanings of arrow keys do not change depending on 'wildoptions'.

src/nvim/fileio.c

@@ -2536,8 +2536,8 @@ int buf_write(buf_T *buf, char *fname, char *sfname, linenr_T start, linenr_T en
       goto fail;
     }
 
-    // Check if the timestamp hasn't changed since reading the file.
-    if (overwriting) {
+    // If 'forceit' is false, check if the timestamp hasn't changed since reading the file.
+    if (overwriting && !forceit) {
       retval = check_mtime(buf, &file_info_old);
       if (retval == FAIL) {
         goto fail;

test/functional/core/fileio_spec.lua

@@ -23,6 +23,8 @@ local iswin = helpers.iswin
 local assert_alive = helpers.assert_alive
 local expect_exit = helpers.expect_exit
 local write_file = helpers.write_file
+local Screen = require('test.functional.ui.screen')
+local feed_command = helpers.feed_command
 local uname = helpers.uname
 
 describe('fileio', function()
@@ -36,6 +38,7 @@ describe('fileio', function()
     os.remove('Xtest_startup_file2')
     os.remove('Xtest_тест.md')
     os.remove('Xtest-u8-int-max')
+    os.remove('Xtest-overwrite-forced')
     rmdir('Xtest_startup_swapdir')
     rmdir('Xtest_backupdir')
   end)
@@ -151,6 +154,61 @@ describe('fileio', function()
     command('edit ++enc=utf32 Xtest-u8-int-max')
     assert_alive()
   end)
+
+  it(':w! does not show "file has been changed" warning', function()
+    clear()
+    write_file("Xtest-overwrite-forced", 'foobar')
+    command('set nofixendofline')
+    local screen = Screen.new(40,4)
+    screen:set_default_attr_ids({
+      [1] = {bold = true, foreground = Screen.colors.Blue1},
+      [2] = {foreground = Screen.colors.Grey100, background = Screen.colors.Red},
+      [3] = {bold = true, foreground = Screen.colors.SeaGreen4}
+    })
+    screen:attach()
+    command("set display-=msgsep shortmess-=F")
+
+    command("e Xtest-overwrite-forced")
+    screen:expect([[
+      ^foobar                                  |
+      {1:~                                       }|
+      {1:~                                       }|
+      "Xtest-overwrite-forced" [noeol] 1L, 6B |
+    ]])
+
+    -- Get current unix time.
+    local cur_unix_time = os.time(os.date("!*t"))
+    local future_time = cur_unix_time + 999999
+    -- Set the file's access/update time to be
+    -- greater than the time at which it was created.
+    local uv = require("luv")
+    uv.fs_utime('Xtest-overwrite-forced', future_time, future_time)
+    -- use async feed_command because nvim basically hangs on the prompt
+    feed_command("w")
+    screen:expect([[
+      {2:WARNING: The file has been changed since}|
+      {2: reading it!!!}                          |
+      {3:Do you really want to write to it (y/n)^?}|
+                                              |
+    ]])
+
+    feed("n")
+    feed("<cr>")
+    screen:expect([[
+      ^foobar                                  |
+      {1:~                                       }|
+      {1:~                                       }|
+                                              |
+    ]])
+    -- Use a screen test because the warning does not set v:errmsg.
+    command("w!")
+    screen:expect([[
+      ^foobar                                  |
+      {1:~                                       }|
+      {1:~                                       }|
+      <erwrite-forced" [noeol] 1L, 6B written |
+    ]])
+  end)
 end)
 
 describe('tmpdir', function()