vim-patch:8.2.4841: empty string considered an error for expand()

Problem:    Empty string considered an error for expand() when 'verbose' is
            set. (Christian Brabandt)
Solution:   Do not give an error for an empty result. (closes vim/vim#10307)
a96edb736d

src/nvim/eval/funcs.c

@@ -1982,7 +1982,7 @@ static void f_expand(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     }
     size_t len;
     char *errormsg = NULL;
-    char_u *result = eval_vars((char_u *)s, (char_u *)s, &len, NULL, &errormsg, NULL);
+    char_u *result = eval_vars((char_u *)s, (char_u *)s, &len, NULL, &errormsg, NULL, false);
     if (p_verbose == 0) {
       emsg_off--;
     } else if (errormsg != NULL) {

src/nvim/ex_docmd.c

@@ -3705,7 +3705,7 @@ int expand_filename(exarg_T *eap, char **cmdlinep, char **errormsgp)
     size_t srclen;
     int escaped;
     char *repl = (char *)eval_vars((char_u *)p, (char_u *)eap->arg, &srclen, &(eap->do_ecmd_lnum),
-                                   errormsgp, &escaped);
+                                   errormsgp, &escaped, true);
     if (*errormsgp != NULL) {           // error detected
       return FAIL;
     }
@@ -6640,12 +6640,13 @@ ssize_t find_cmdline_var(const char_u *src, size_t *usedlen)
 /// @param lnump           line number for :e command, or NULL
 /// @param errormsg        pointer to error message
 /// @param escaped         return value has escaped white space (can be NULL)
+/// @param empty_is_error  empty result is considered an error
 ///
 /// @return          an allocated string if a valid match was found.
 ///                  Returns NULL if no match was found.  "usedlen" then still contains the
 ///                  number of characters to skip.
 char_u *eval_vars(char_u *src, char_u *srcstart, size_t *usedlen, linenr_T *lnump, char **errormsg,
-                  int *escaped)
+                  int *escaped, bool empty_is_error)
 {
   char *result;
   char *resultbuf = NULL;
@@ -6890,7 +6891,7 @@ char_u *eval_vars(char_u *src, char_u *srcstart, size_t *usedlen, linenr_T *lnum
     }
   }
 
-  if (resultlen == 0 || valid != VALID_HEAD + VALID_PATH) {
+  if (empty_is_error && (resultlen == 0 || valid != VALID_HEAD + VALID_PATH)) {
     if (valid != VALID_HEAD + VALID_PATH) {
       // xgettext:no-c-format
       *errormsg = _("E499: Empty file name for '%' or '#', only works with \":p:h\"");
@@ -6919,7 +6920,8 @@ char *expand_sfile(char *arg)
       // replace "<sfile>" with the sourced file name, and do ":" stuff
       size_t srclen;
       char *errormsg;
-      char *repl = (char *)eval_vars((char_u *)p, (char_u *)result, &srclen, NULL, &errormsg, NULL);
+      char *repl = (char *)eval_vars((char_u *)p, (char_u *)result, &srclen, NULL, &errormsg, NULL,
+                                     true);
       if (errormsg != NULL) {
         if (*errormsg) {
           emsg(errormsg);

src/nvim/path.c

@@ -2135,7 +2135,8 @@ int expand_wildcards_eval(char_u **pat, int *num_file, char ***file, int flags)
 
   if (*exp_pat == '%' || *exp_pat == '#' || *exp_pat == '<') {
     emsg_off++;
-    eval_pat = eval_vars((char_u *)exp_pat, (char_u *)exp_pat, &usedlen, NULL, &ignored_msg, NULL);
+    eval_pat = eval_vars((char_u *)exp_pat, (char_u *)exp_pat, &usedlen, NULL, &ignored_msg, NULL,
+                         true);
     emsg_off--;
     if (eval_pat != NULL) {
       exp_pat = (char *)concat_str(eval_pat, (char_u *)exp_pat + usedlen);

src/nvim/testdir/test_expand_func.vim

@@ -107,10 +107,14 @@ endfunc
 
 func Test_expand()
   new
-  call assert_equal("",  expand('%:S'))
+  call assert_equal("''", expand('%:S'))
   call assert_equal('3', '<slnum>'->expand())
   call assert_equal(['4'], expand('<slnum>', v:false, v:true))
   " Don't add any line above this, otherwise <slnum> will change.
+  call assert_equal("", expand('%'))
+  set verbose=1
+  call assert_equal("", expand('%'))
+  set verbose=0
   quit
 endfunc