From 0ef416e7edc4c66a06a3c05025720dfc98cf07f5 Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Thu, 3 Feb 2022 16:23:24 -0500
Subject: [PATCH] c++: add comment

gcc/cp/ChangeLog:

	* pt.cc (iterative_hash_template_arg): Add comment.
---
 gcc/cp/pt.cc | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 60641d89c193..f6406126b5b9 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -1797,6 +1797,10 @@ iterative_hash_template_arg (tree arg, hashval_t val)
   switch (code)
     {
     case ARGUMENT_PACK_SELECT:
+      /* Getting here with an ARGUMENT_PACK_SELECT means we're probably
+	 preserving it in a hash table, which is bad because it will change
+	 meaning when gen_elem_of_pack_expansion_instantiation changes the
+	 ARGUMENT_PACK_SELECT_INDEX.  */
       gcc_unreachable ();
 
     case ERROR_MARK:
-- 
2.47.2