[Bug c++/30897] [4.1/4.2/4.3 regression] ICE with default argument in template template parameter

["mmitchel at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #2 from mmitchel at gcc dot gnu dot org  2007-03-23 00:50 -------
This code in lookup_template_class:

-      /* Consider an example where a template template parameter declared as
-
-          template <class T, class U = std::allocator<T> > class TT
-
-        The template parameter level of T and U are one level larger than
-        of TT.  To proper process the default argument of U, say when an
-        instantiation `TT<int>' is seen, we need to build the full
-        arguments containing {int} as the innermost level.  Outer levels,
-        available when not appearing as default template argument, can be
-        obtained from `current_template_args ()'.
-
-        Suppose that TT is later substituted with std::vector.  The above
-        instantiation is `TT<int, std::allocator<T> >' with TT at
-        level 1, and T at level 2, while the template arguments at level 1
-        becomes {std::vector} and the inner level 2 is {int}.  */
-
-      if (current_template_parms)
-       arglist = add_to_template_args (current_template_args (), arglist);

is simply bogus; the current template arguments have nothing to do with the
situation.  It is this code which causes the crash.

I believe that the correct change is to remove that code and to make template
parameters for template template parameters restart at level 1.  So, in:

  template <template <typename T> class A> 
  class B {};

both T and A have level 1.  (Right now, T has level 2.)

T should have level 1 because, when instantiating A, that's what make sense.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30897