patch 8.2.4168: disallowing empty function name breaks existing plugins

Problem:    Disallowing empty function name breaks existing plugins.
Solution:   Allow empty function name in legacy script.

src/testdir/sautest/autoload/foo.vim

@@ -9,3 +9,7 @@ endfunc
 func foo#addFoo(head)
   return a:head .. 'foo'
 endfunc
+
+func foo#()
+  return 'empty'
+endfunc

src/testdir/test_autoload.vim

@@ -10,6 +10,9 @@ func Test_autoload_dict_func()
   call assert_equal(1, g:called_foo_bar_echo)
 
   eval 'bar'->g:foo#addFoo()->assert_equal('barfoo')
+
+  " empty name works in legacy script
+  call assert_equal('empty', foo#())
 endfunc
 
 func Test_source_autoload()
@@ -23,4 +26,5 @@ func Test_autoload_vim9script()
   call assert_equal(49, auto9#add42(7))
 endfunc
 
+
 " vim: shiftwidth=2 sts=2 expandtab

src/userfunc.c

@@ -3693,8 +3693,8 @@ trans_function_name(
     // Note that TFN_ flags use the same values as GLV_ flags.
     end = get_lval(start, NULL, &lv, FALSE, skip, flags | GLV_READ_ONLY,
 					      lead > 2 ? 0 : FNE_CHECK_START);
-    if (end == start
+    if (end == start || (in_vim9script() && end != NULL
-		   || (end != NULL && end[-1] == AUTOLOAD_CHAR && *end == '('))
+				   && end[-1] == AUTOLOAD_CHAR && *end == '('))
     {
 	if (!skip)
 	    emsg(_(e_function_name_required));

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4168,
 /**/
     4167,
 /**/