[Bug c++/51239] New: ICE with variadic template alias

["illissius at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51239

             Bug #: 51239
           Summary: ICE with variadic template alias
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: illissius@gmail.com


The following code (self-contained):

template<typename T, typename...>
using head = T;

template<typename... Ts>
using x = head<Ts...>;

results for me in the following error:

$ LD_LIBRARY_PATH=/opt/gcc4.7/lib:$LD_LIBRARY_PATH PATH=/opt/gcc4.7/bin/:$PATH
g++ -std=c++11 test68.cpp 
test68.cpp:2:15: internal compiler error: tree check: accessed elt 2 of
tree_vec with 1 elts in tsubst_pack_expansion, at cp/pt.c:9298
Please submit a full bug report,
with preprocessed source if appropriate.
See <http://gcc.gnu.org/bugs.html> for instructions.

GCC version:

$ LD_LIBRARY_PATH=/opt/gcc4.7/lib:$LD_LIBRARY_PATH PATH=/opt/gcc4.7/bin/:$PATH
g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/opt/gcc4.7/libexec/gcc/x86_64-unknown-linux-gnu/4.7.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../trunk/configure --disable-bootstrap
--enable-languages=c,c++ --prefix=/opt/gcc4.7
Thread model: posix
gcc version 4.7.0 20111120 (experimental) (GCC)

SVN revision: 181530

A version from 20111109 produced the same error (at a different line number).

The line in question in the current revision, and the two lines before it:
          levels = TMPL_ARGS_DEPTH (args);
          if (level <= levels)
            arg_pack = TMPL_ARG (args, level, idx);