vim-patch:8.0.1231: expanding file name drops dash

Problem:    Expanding file name drops dash. (stucki)
Solution:   Use the right position. (Christian Brabandt, closes vim/vim#2184)
c312b8b87a

src/nvim/ex_docmd.c

@@ -8521,9 +8521,13 @@ eval_vars (
       if (*s == '<')                    /* "#<99" uses v:oldfiles */
         ++s;
       i = getdigits_int(&s);
+      if (s == src + 2 && src[1] == '-') {
+        // just a minus sign, don't skip over it
+        s--;
+      }
       *usedlen = (size_t)(s - src);           /* length of what we expand */
 
-      if (src[1] == '<') {
+      if (src[1] == '<' && i != 0) {
         if (*usedlen < 2) {
           /* Should we give an error message for #<text? */
           *usedlen = 1;
@@ -8536,6 +8540,9 @@ eval_vars (
           return NULL;
         }
       } else {
+        if (i == 0 && src[1] == '<' && *usedlen > 1) {
+          *usedlen = 1;
+        }
         buf = buflist_findnr(i);
         if (buf == NULL) {
           *errormsg = (char_u *)_(

src/nvim/testdir/test_cmdline.vim

@@ -380,6 +380,27 @@ 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