Hi,

over the last few days I spent some time on this regression, which at 
first seemed just a minor error-recovery issue, but then I noticed that 
very slightly tweeking the original testcase uncovered a pretty serious 
ICE on valid:

template<typename SX, typename ...XE> void
fk (XE..., int/*SW*/);

void
w9 (void)
{
   fk<int> (0);
}

The regression has to do with the changes committed by Jason for 
c++/86932, in particular with the condition in coerce_template_parms:

    if (template_parameter_pack_p (TREE_VALUE (parm))
       && (arg || !(complain & tf_partial))
       && !(arg && ARGUMENT_PACK_P (arg)))

which has the additional (arg || !complain & tf_partial)) false for the 
present testcase, thus the null arg is not changed into an empty pack, 
thus later  instantiate_template calls check_instantiated_args which 
finds it still null and crashes. Now, likely some additional analysis is 
in order, but for sure there is an important difference between the 
testcase which came with c++/86932 and the above: non-type vs type 
template parameter pack. It seems to me that the kind of problem fixed 
in c++/86932 cannot occur with type packs, because it boils down to a 
reference to a previous parm (full disclosure: the comments and logic in 
fixed_parameter_pack_p helped me a lot here). Thus I had the idea of 
simply restricting the scope of the new condition above by adding an || 
TREE_CODE (TREE_VALUE (parm)) == TYPE_DECL, which definitely leads to a 
clean testsuite and a proper behavior on the new testcases, AFAICS. I'm 
attaching what I tested on x86_64-linux.

Thanks, Paolo.

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