c++: add comment

gcc/cp/ChangeLog:

	* pt.cc (iterative_hash_template_arg): Add comment.

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 60641d8..f640612 100644 (file)
--- 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: