[C++ Patch/RFC] PR 89900 ("[9 Regression] ICE: Segmentation fault (in check_instantiated_arg)")

[Jason Merrill]

On 4/11/19 11:20 AM, Paolo Carlini wrote:
> 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)
> {
>  Â  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:
> 
>  Â Â  if (template_parameter_pack_p (TREE_VALUE (parm))
>  Â Â Â  Â  && (arg || !(complain & tf_partial))
>  Â Â Â  Â  && !(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.

I think the important property here is that it's non-terminal, not that 
it's a type pack.  We can't deduce anything for a non-terminal pack, so 
we should go ahead and make an empty pack.

Jason