fix(eval): checking for a non-empty string is too strict (#15987)

Cherry-pick check_for_nonempty_string() from patch vim-8.2.2133 and
apply it on the bases of https://github.com/neovim/neovim/pull/13489

https://github.com/vim/vim/commit/2a9d5d386bea8455b37c1accebc45683ec51d6fb

3 files changed, 21 insertions, 7 deletions

diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 1c78e25639..d0f6c178df 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -2201,7 +2201,7 @@ static void f_win_execute(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 /// "exepath()" function
 static void f_exepath(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
-  if (tv_check_for_string(&argvars[0]) == FAIL) {
+  if (tv_check_for_nonempty_string(&argvars[0]) == FAIL) {
     return;
   }
 
@@ -2661,9 +2661,9 @@ static void f_fnamemodify(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   char buf[NUMBUFLEN];
   const char *fname = tv_get_string_chk(&argvars[0]);
   const char *const mods = tv_get_string_buf_chk(&argvars[1], buf);
-  if (fname == NULL || mods == NULL) {
+  if (fname == NULL) {
     fname = NULL;
-  } else {
+  } else if (mods != NULL && *mods != NUL) {
     len = strlen(fname);
     size_t usedlen = 0;
     if (*mods != NUL) {
diff --git a/src/nvim/eval/typval.c b/src/nvim/eval/typval.c
index 6300ce6150..4e845cffdd 100644
--- a/src/nvim/eval/typval.c
+++ b/src/nvim/eval/typval.c
@@ -3146,19 +3146,31 @@ float_T tv_get_float(const typval_T *const tv)
   return 0;
 }
 
-// Give an error and return FAIL unless "tv" is a non-empty string.
+// Give an error and return FAIL unless "tv" is a string.
 int tv_check_for_string(const typval_T *const tv)
   FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT FUNC_ATTR_PURE
 {
-  if (tv->v_type != VAR_STRING
-      || tv->vval.v_string == NULL
-      || *tv->vval.v_string == NUL) {
+  if (tv->v_type != VAR_STRING) {
     EMSG(_(e_stringreq));
     return FAIL;
   }
   return OK;
 }
 
+// Give an error and return FAIL unless "tv" is a non-empty string.
+int tv_check_for_nonempty_string(const typval_T *const tv)
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT FUNC_ATTR_PURE
+{
+  if (tv_check_for_string(tv) == FAIL) {
+      return FAIL;
+  }
+  if (tv->vval.v_string == NULL || *tv->vval.v_string == NUL) {
+    EMSG(_(e_non_empty_string_required));
+    return FAIL;
+  }
+  return OK;
+}
+
 /// Get the string value of a "stringish" VimL object.
 ///
 /// @param[in]  tv  Object to get value of.
diff --git a/src/nvim/globals.h b/src/nvim/globals.h
index d1f6e2dbd9..21e186ec8c 100644
--- a/src/nvim/globals.h
+++ b/src/nvim/globals.h
@@ -994,6 +994,8 @@ EXTERN char_u e_floatonly[] INIT(=N_(
                                     "E5601: Cannot close window, only floating window would remain"));
 EXTERN char_u e_floatexchange[] INIT(=N_("E5602: Cannot exchange or rotate float"));
 
+EXTERN char e_non_empty_string_required[] INIT(= N_("E1142: Non-empty string required"));
+
 EXTERN char e_cannot_define_autocommands_for_all_events[] INIT(=
                                                                  N_(
                                                                    "E1155: Cannot define autocommands for ALL events"));