patch 8.2.0403: when 'buftype' is "nofile" there is no overwrite check

Problem:    When 'buftype' is "nofile" there is no overwrite check.
Solution:   Also check for existing file when 'buftype' is set.
            (closes #5807)

src/ex_cmds.c

@@ -2074,8 +2074,8 @@ check_overwrite(
     int		other)	    // writing under other name
 {
     /*
-     * write to other file or b_flags set or not writing the whole file:
-     * overwriting only allowed with '!'
+     * Write to another file or b_flags set or not writing the whole file:
+     * overwriting only allowed with '!'.
      */
     if (       (other
 		|| (buf->b_flags & BF_NOTEDITED)
@@ -2083,9 +2083,6 @@ check_overwrite(
 		    && vim_strchr(p_cpo, CPO_OVERNEW) == NULL)
 		|| (buf->b_flags & BF_READERR))
 	    && !p_wa
-#ifdef FEAT_QUICKFIX
-	    && !bt_nofilename(buf)
-#endif
 	    && vim_fexists(ffname))
     {
 	if (!eap->forceit && !eap->append)

src/testdir/test_options.vim

@@ -656,7 +656,15 @@ func Test_buftype()
   call setline(1, ['L1'])
   set buftype=nowrite
   call assert_fails('write', 'E382:')
-  close!
+
+  for val in ['', 'nofile', 'nowrite', 'acwrite', 'quickfix', 'help', 'terminal', 'prompt', 'popup']
+    exe 'set buftype=' .. val
+    call writefile(['something'], 'XBuftype')
+    call assert_fails('write XBuftype', 'E13:', 'with buftype=' .. val)
+  endfor
+
+  call delete('XBuftype')
+  bwipe!
 endfunc
 
 " Test for the 'shellquote' option

src/version.c

@@ -738,6 +738,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    403,
 /**/
     402,
 /**/