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

[Patrick Palka]

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))
            {
              champ = nullptr;
              break;
diff --git a/gcc/testsuite/g++.dg/overload/error7.C 
b/gcc/testsuite/g++.dg/overload/error7.C
new file mode 100644
index 00000000000..68aaa236de4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/overload/error7.C
@@ -0,0 +1,10 @@
+// PR c++/115239
+
+bool foo(const char *, char *, long);     // #1, strictly viable, ambig with #2
+bool foo(const char *, char *, unsigned); // #2, strictly viable, ambig with #1
+bool foo(char, char, long);               // #3, non-strictly viable
+bool foo(char, char, unsigned);           // #4, non-strictly viable
+
+int main() {
+  foo((char *)0, (char *)0, 0); // { dg-error "ambiguous" }
+}
-- 
2.45.2.457.g8d94cfb545