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.

gcc/cp/call.c

@@ -8685,19 +8685,21 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
 	    return error_mark_node;
 	}
 
-      /* Optimize away vtable lookup if we know that this
-	 function can't be overridden.  We need to check if
-	 the context and the type where we found fn are the same,
-	 actually FN might be defined in a different class
-	 type because of a using-declaration. In this case, we
-	 do not want to perform a non-virtual call.  Note that
-	 resolves_to_fixed_type_p checks CLASSTYPE_FINAL too.  */
+      /* See if the function member or the whole class type is declared
+	 final and the call can be devirtualized.  */
       if (DECL_FINAL_P (fn)
-	  || (resolves_to_fixed_type_p (arg, 0)
-	      && same_type_ignoring_top_level_qualifiers_p
-	      (DECL_CONTEXT (fn), BINFO_TYPE (cand->conversion_path)))) 
+	  || CLASSTYPE_FINAL (TYPE_METHOD_BASETYPE (TREE_TYPE (fn))))
 	flags |= LOOKUP_NONVIRTUAL;
 
+      /* If we know the dynamic type of the object, look up the final overrider
+	 in the BINFO.  */
+      if (DECL_VINDEX (fn) && (flags & LOOKUP_NONVIRTUAL) == 0
+	  && resolves_to_fixed_type_p (arg))
+	{
+	  fn = lookup_vfn_in_binfo (DECL_VINDEX (fn), cand->conversion_path);
+	  flags |= LOOKUP_NONVIRTUAL;
+	}
+
       /* [class.mfct.nonstatic]: If a nonstatic member function of a class
 	 X is called for an object that is not of type X, or of a type
 	 derived from X, the behavior is undefined.

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

gcc/cp/cp-tree.h

@@ -6401,7 +6401,7 @@ extern tree outermost_open_class		(void);
 extern tree current_nonlambda_class_type	(void);
 extern tree finish_struct			(tree, tree);
 extern void finish_struct_1			(tree);
-extern int resolves_to_fixed_type_p		(tree, int *);
+extern int resolves_to_fixed_type_p		(tree, int * = NULL);
 extern void init_class_processing		(void);
 extern int is_empty_class			(tree);
 extern bool is_really_empty_class		(tree, bool);
@@ -6415,6 +6415,7 @@ extern void pop_lang_context			(void);
 extern tree instantiate_type			(tree, tree, tsubst_flags_t);
 extern void build_self_reference		(void);
 extern int same_signature_p			(const_tree, const_tree);
+extern tree lookup_vfn_in_binfo			(tree, tree);
 extern void maybe_add_class_template_decl_list	(tree, tree, int);
 extern void unreverse_member_declarations	(tree);
 extern void invalidate_class_lookup_cache	(void);

gcc/testsuite/g++.dg/template/virtual5.C

+// PR c++/95158
+// { dg-do run }
+
+class Base {
+    public:
+        virtual void foo()=0;
+};
+
+template <typename T>
+class MiddleA : virtual public Base {
+    public:
+        virtual void foo() {}
+};
+
+class MiddleB : virtual public Base {};
+
+template <typename T>
+class Derived : public MiddleA<T>, public MiddleB {
+    public:
+        void bar()
+        {
+	  Derived d;
+	  d.foo();
+        }
+};
+
+int main()
+{
+  Derived<void> a;
+  a.bar(); // Instantiate the template
+}