[PATCH] c++/68711 - [5 regression] SEGV on an invalid offsetof of a member of a virtual base

[Martin Sebor <msebor@gmail.com>]

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The attached patch restores the "invalid access to non-static
data member" error for invalid offsetof-like expressions
involving members of virtually derived base classes that was
inadvertently made ineffective as a result of the merge of
the C++ delayed folding branch.

Martin

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gcc/testsuite/ChangeLog:
2015-12-05  Martin Sebor  <msebor@redhat.com>

	PR c++/68711
	* g++.dg/other/offsetof8.C: New test.

gcc/cp/ChangeLog:
2015-12-05  Martin Sebor  <msebor@redhat.com>

	PR c++/68711
	* typeck.c (build_class_member_access_expr): Strip NOPs before
        testing a potentially null operand for equality to zero.

Index: gcc/cp/typeck.c
===================================================================
--- gcc/cp/typeck.c	(revision 231314)
+++ gcc/cp/typeck.c	(working copy)
@@ -2358,8 +2358,14 @@
       int type_quals;
       tree member_type;
 
-      null_object_p = (INDIRECT_REF_P (object)
-		       && integer_zerop (TREE_OPERAND (object, 0)));
+      if (INDIRECT_REF_P (object))
+	{
+	  tree oper = TREE_OPERAND (object, 0);
+	  STRIP_NOPS (oper);
+	  null_object_p = integer_zerop (oper);
+	}
+      else
+	null_object_p = false;
 
       /* Convert OBJECT to the type of MEMBER.  */
       if (!same_type_p (TYPE_MAIN_VARIANT (object_type),
Index: gcc/testsuite/g++.dg/other/offsetof8.C
===================================================================
--- gcc/testsuite/g++.dg/other/offsetof8.C	(revision 0)
+++ gcc/testsuite/g++.dg/other/offsetof8.C	(working copy)
@@ -0,0 +1,12 @@
+// PR c++/68711 - [5 regression] SEGV on an invalid offsetof of a member
+//                of a virtual base
+// { dg-do compile }
+
+struct A { int i; };
+
+struct B: virtual A { };
+
+int a[]  = {
+  !&((B*)0)->i,    // { dg-error "invalid access to non-static data member" }
+  __builtin_offsetof (B, i)   // { dg-error "invalid access to non-static" }
+};