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

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 second arg conversion since the first arg conversion was deemed
bad, and this trips up joust when called on #3 and #4 which assumes all
arg conversions are there.

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.

Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit 7fed7e9bbc57d502e141e079a6be2706bdbd4560)

gcc/cp/call.cc

@@ -13488,7 +13488,8 @@ tourney (struct z_candidate *candidates, tsubst_flags_t complain)
 	{
 	  previous_worse_champ = nullptr;
 	  champ = &(*challenger)->next;
-	  if (!*champ || !(*champ)->viable)
+	  if (!*champ || !(*champ)->viable
+	      || (*champ)->viable < (*challenger)->viable)
 	    {
 	      champ = nullptr;
 	      break;

gcc/testsuite/g++.dg/overload/error7.C

@@ -0,0 +1,10 @@
+// PR c++/115239
+
+bool foo(char *, long);     // #1, strictly viable, ambig with #2
+bool foo(char *, unsigned); // #2, strictly viable, ambig with #1
+bool foo(char, long);       // #3, non-strictly viable
+bool foo(char, unsigned);   // #4, non-strictly viable
+
+int main() {
+  foo((char *)0, 0); // { dg-error "ambiguous" }
+}