Re: [PATCH] c++: ICE w/ ambig and non-strictly-viable cands [PR115239]

[Jason Merrill <jason@redhat.com>]

On 6/12/24 13:56, Patrick Palka wrote:
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk/14?
> 
> -- >8 --
> 
> Here during overload resolution we have two strictly viable ambiguous
> candidates #1 and #2, and two non-strictly viable candidates #3 and #4
> which we hold on to ever since r14-6522.  These latter candidates have
> an empty third arg conversion since the second arg conversion was deemed
> bad.  This ends up causing an ICE during joust for #3 and #4 due to this
> empty arg conversion.
> 
> We can fix this by making joust robust to empty arg conversions, but in
> this situation we shouldn't need to compare #3 and #4 at all given that
> we have a strictly viable candidate.  To that end, this patch makes
> tourney shortcut considering non-strictly viable candidates upon
> encountering ambiguity between two strictly viable candidates, taking
> advantage of the fact that the candidates list is sorted according to
> viability via splice_viable.
> 
> 	PR c++/115239
> 
> gcc/cp/ChangeLog:
> 
> 	* call.cc (tourney): Don't consider a non-strictly viable
> 	candidate as the champ if there was ambiguity between two
> 	strictly viable candidates.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/overload/error7.C: New test.
> ---
>   gcc/cp/call.cc                         |  4 +++-
>   gcc/testsuite/g++.dg/overload/error7.C | 10 ++++++++++
>   2 files changed, 13 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/overload/error7.C
> 
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index ed68eb3c568..82c70f5c39f 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -13484,9 +13484,11 @@ tourney (struct z_candidate *candidates, tsubst_flags_t complain)
>   	}
>         else
>   	{
> +	  z_candidate *prev_champ = *champ;
>   	  previous_worse_champ = nullptr;
>   	  champ = &(*challenger)->next;
> -	  if (!*champ || !(*champ)->viable)
> +	  if (!*champ || !(*champ)->viable
> +	      || (prev_champ->viable == 1 && (*champ)->viable == -1))

Maybe

(!*champ || (*champ)->viable < prev_champ->viable) ?

OK with that change.

Jason