From de31f5445b12fd9ab9969dc536d821fe6f0edad0 Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Mon, 21 Jun 2021 07:54:26 -0400
Subject: [PATCH] c++: conversion to base of vbase in NSDMI [PR80431]

The delayed processing of conversions to a virtual base in an NSDMI
assumes the target base type is a (possibly indirect) virtual base of
the current class, but the target base type could also be a base of a
virtual base, as in the testcase below.  Since such a base isn't a part
of CLASSTYPE_VBASECLASSES, we end up miscompiling the testcase due to
the call to build_base_path (with binfo=NULL_TREE) silently returning
error_mark_node.  Fix this by using convert_to_base to build the
conversion instead.

	PR c++/80431

gcc/cp/ChangeLog:

	* tree.c (bot_replace): Use convert_to_base to build the
	conversion to the (morally) virtual base.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/nsdmi-virtual1a.C: New test.
---
 gcc/cp/tree.c                                | 14 ++++------
 gcc/testsuite/g++.dg/cpp0x/nsdmi-virtual1a.C | 28 ++++++++++++++++++++
 2 files changed, 33 insertions(+), 9 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/nsdmi-virtual1a.C

diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index fec5afaa2be..297da2b1550 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -3242,15 +3242,11 @@ bot_replace (tree* t, int* /*walk_subtrees*/, void* data_)
   else if (TREE_CODE (*t) == CONVERT_EXPR
 	   && CONVERT_EXPR_VBASE_PATH (*t))
     {
-      /* In an NSDMI build_base_path defers building conversions to virtual
-	 bases, and we handle it here.  */
-      tree basetype = TYPE_MAIN_VARIANT (TREE_TYPE (TREE_TYPE (*t)));
-      vec<tree, va_gc> *vbases = CLASSTYPE_VBASECLASSES (current_class_type);
-      int i; tree binfo;
-      FOR_EACH_VEC_SAFE_ELT (vbases, i, binfo)
-	if (BINFO_TYPE (binfo) == basetype)
-	  break;
-      *t = build_base_path (PLUS_EXPR, TREE_OPERAND (*t, 0), binfo, true,
+      /* In an NSDMI build_base_path defers building conversions to morally
+	 virtual bases, and we handle it here.  */
+      tree basetype = TREE_TYPE (*t);
+      *t = convert_to_base (TREE_OPERAND (*t, 0), basetype,
+			    /*check_access=*/false, /*nonnull=*/true,
 			    tf_warning_or_error);
     }
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/nsdmi-virtual1a.C b/gcc/testsuite/g++.dg/cpp0x/nsdmi-virtual1a.C
new file mode 100644
index 00000000000..dc847cc16e5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/nsdmi-virtual1a.C
@@ -0,0 +1,28 @@
+// PR c++/80431
+// { dg-do run { target c++11 } }
+
+// A variant of nsdmi-virtual1.C where A is only a morally virtual base of B.
+
+struct A
+{
+  A(): i(42) { }
+  int i;
+  int f() { return i; }
+};
+
+struct D : A { int pad; };
+
+struct B : virtual D
+{
+  int j = i + f();
+  int k = A::i + A::f();
+};
+
+struct C: B { int pad; };
+
+int main()
+{
+  C c;
+  if (c.j != 84 || c.k != 84)
+    __builtin_abort();
+}
-- 
2.31.1