patch 8.2.3031: no error if a function name starts with an underscore Commit: https://github.com/vim/vim/commit/22f17a29cd0b0cc3107dc6cd1d96c62eee52a7d9 Author: Bram Moolenaar <Bram@vim.org> Date: Mon Jun 21 20:48:58 2021 +0200 patch 8.2.3031: no error if a function name starts with an underscore Problem: No error if a function name starts with an underscore. (Naohiro Ono) Solution: In Vim9 script disallow a function name starting with an underscore, as is mentioned in the help. (closes #8414)

--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -3595,7 +3595,8 @@ trans_function_name(
 		lead += (int)STRLEN(sid_buf);
 	}
     }
-    else if (!(flags & TFN_INT) && builtin_function(lv.ll_name, len))
+    else if (!(flags & TFN_INT) && (builtin_function(lv.ll_name, len)
+				   || (in_vim9script() && *lv.ll_name == '_')))
     {
 	semsg(_("E128: Function name must start with a capital or \"s:\": %s"),
 								       start);