[Bug c++/84796] ICE in a template parameter pack expansion

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84796

David Malcolm <dmalcolm at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2018-03-12
                 CC|                            |dmalcolm at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Thanks for filing this bug.

Confirmed.  The bogus access is here in tsubst_pack_expansion:

11742             if (level <= levels)
11743               arg_pack = TMPL_ARG (args, level, idx);

(gdb) call debug_tree (args)
 <tree_vec 0x7ffff1a24000 length:0>
(gdb) p level
$5 = 1
(gdb) p idx
$6 = 0
(gdb) p levels
$7 = 1

Bisection with checked binaries shows this started with r208178; I'm able to
reproduce it with a checked build of gcc 7; it's only failing to show up for
regular gcc 7 builds since checking is turned off by default for release
builds.