c++: Fix FE devirt with diamond inheritance [PR95158]

This started breaking in GCC 8 because of the fix for PR15272; after that
change, we (correctly) remember the lookup from template parsing time that
found Base::foo through the non-dependent MiddleB base, and so we overlook
the overrider in MiddleA.  But given that, the devirtualization condition
from the fix for PR59031 is insufficient; we know that d has to be a
Derived, and we found Base::foo in Base, but forcing a non-virtual call
gets the wrong function.

Fixed by removing the PR59031 code that the PR67184 patch moved to
build_over_call, and instead looking up the overrider in BINFO_VIRTUALS.

gcc/cp/ChangeLog:

	PR c++/95158
	* class.c (lookup_vfn_in_binfo): New.
	* call.c (build_over_call): Use it.
	* cp-tree.h (resolves_to_fixed_type_p): Add default argument.
	(lookup_vfn_in_binfo): Declare.

gcc/testsuite/ChangeLog:

	PR c++/95158
	* g++.dg/template/virtual5.C: New test.

1 files changed, 14 insertions, 0 deletions

diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index 757e010..f8e38ec 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -2445,6 +2445,20 @@ get_vcall_index (tree fn, tree type)
   gcc_unreachable ();
 }
 
+/* Given a DECL_VINDEX of a virtual function found in BINFO, return the final
+   overrider at that index in the vtable.  This should only be used when we
+   know that BINFO is correct for the dynamic type of the object.  */
+
+tree
+lookup_vfn_in_binfo (tree idx, tree binfo)
+{
+  int ix = tree_to_shwi (idx);
+  if (TARGET_VTABLE_USES_DESCRIPTORS)
+    ix /= MAX (TARGET_VTABLE_USES_DESCRIPTORS, 1);
+  tree virtuals = BINFO_VIRTUALS (binfo);
+  return TREE_VALUE (chain_index (ix, virtuals));
+}
+
 /* Update an entry in the vtable for BINFO, which is in the hierarchy
    dominated by T.  FN is the old function; VIRTUALS points to the
    corresponding position in the new BINFO_VIRTUALS list.  IX is the index