Hide internal IDs

* parse.y (internal_id): number the ID serial for internal use by
  counting down from the neary maximum value, not to accidentally
  match permanent IDs.

[Bug #15768]

1 files changed, 2 insertions, 1 deletions

diff --git a/parse.y b/parse.y
index c769ffe485..7c5e8c3772 100644
--- a/parse.y
+++ b/parse.y
@@ -11812,8 +11812,9 @@ rb_init_parse(void)
 static ID
 internal_id(struct parser_params *p)
 {
+    const ID max_id = RB_ID_SERIAL_MAX & ~0xffff;
     ID id = (ID)vtable_size(p->lvtbl->args) + (ID)vtable_size(p->lvtbl->vars);
-    id += ((tLAST_TOKEN - ID_INTERNAL) >> ID_SCOPE_SHIFT) + 1;
+    id = max_id - id;
     return ID_STATIC_SYM | ID_INTERNAL | (id << ID_SCOPE_SHIFT);
 }
 #endif /* !RIPPER */