[Bug preprocessor/47311] [4.6 Regression][C++0x] ICE in tsubst @cp/pt.c:10502

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47311

--- Comment #23 from Dodji Seketeli <dodji at gcc dot gnu.org> 2011-01-20 10:41:07 UTC ---
Here is what I understand from this issue.

In this example:

template < typename > class A;
template <class Key, class T, template < typename TF = T> class TC = A> class
B;

1/ During the fixup of the default argument T of the parameter TF of the
template template parm TC, we need to build the full list of implicit
arguments we have so far which {[Key, T], TF}. [Key, T] are made from the
template parameters of B. Note that [Key, T] are not fixed up yet
because they will be fixed up when we have fully parsed all the
parameters of B.

2/ That list {[Key, T], TF} is to be substituted into the default
argument of TF, that happens to be T.

The problem is at that point in fixup_template_parms, we don't have
access to [KEY, T]. current_template_args returns {[NULL], TF}. Hence
the crash in the substituting of 2/

I think no fixup should happen in 1/ as we should probably wait until
the end of parsing the parms of B to fixup all it template parms at
once.