patch 8.0.1231: expanding file name drops dash

Problem:    Expanding file name drops dash. (stucki)
Solution:   Use the right position. (Christian Brabandt, closes #2184)

src/ex_docmd.c

@@ -10715,9 +10715,12 @@ eval_vars(
 		if (*s == '<')		/* "#<99" uses v:oldfiles */
 		    ++s;
 		i = (int)getdigits(&s);
+		if (s == src + 2 && src[1] == '-')
+		    /* just a minus sign, don't skip over it */
+		    s--;
 		*usedlen = (int)(s - src); /* length of what we expand */
 
-		if (src[1] == '<')
+		if (src[1] == '<' && i != 0)
 		{
 		    if (*usedlen < 2)
 		    {
@@ -10740,6 +10743,8 @@ eval_vars(
 		}
 		else
 		{
+		    if (i == 0 && src[1] == '<' && *usedlen > 1)
+			*usedlen = 1;
 		    buf = buflist_findnr(i);
 		    if (buf == NULL)
 		    {

src/testdir/test_cmdline.vim

@@ -365,6 +365,27 @@ func Test_cmdline_complete_user_cmd()
   delcommand Foo
 endfunc
 
+func Test_cmdline_write_alternatefile()
+  new
+  call setline('.', ['one', 'two'])
+  f foo.txt
+  new
+  f #-A
+  call assert_equal('foo.txt-A', expand('%'))
+  f #<-B.txt
+  call assert_equal('foo-B.txt', expand('%'))
+  f %<
+  call assert_equal('foo-B', expand('%'))
+  new
+  call assert_fails('f #<', 'E95')
+  bw!
+  f foo-B.txt
+  f %<-A
+  call assert_equal('foo-B-A', expand('%'))
+  bw!
+  bw!
+endfunc
+
 " using a leading backslash here
 set cpo+=C
 

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1231,
 /**/
     1230,
 /**/