Index: test/SemaCXX/attr-nonnull.cpp
===================================================================
--- test/SemaCXX/attr-nonnull.cpp	(revision 173218)
+++ test/SemaCXX/attr-nonnull.cpp	(working copy)
@@ -31,3 +31,12 @@
     f2(0, 0); // expected-warning{{null passed to a callee which requires a non-null argument}}
   }
 }
+
+namespace test3 {
+__attribute__((nonnull(1))) void f(void *ptr);
+
+void g() {
+  f(static_cast<char*>((void*)0));  // expected-warning{{null passed}}
+  f(static_cast<char*>(0));  // expected-warning{{null passed}}
+}
+}
Index: lib/Sema/SemaChecking.cpp
===================================================================
--- lib/Sema/SemaChecking.cpp	(revision 173218)
+++ lib/Sema/SemaChecking.cpp	(working copy)
@@ -1837,8 +1837,20 @@
                                   e = NonNull->args_end();
        i != e; ++i) {
     const Expr *ArgExpr = ExprArgs[*i];
-    if (ArgExpr->isNullPointerConstant(Context,
-                                       Expr::NPC_ValueDependentIsNotNull))
+
+    // As a special case, transparent unions initialied with zero are considered
+    // null for the purposes of the nonnull attribute.
+    if (const RecordType *UT = ArgExpr->getType()->getAsUnionType()) {
+      if (UT->getDecl()->hasAttr<TransparentUnionAttr>())
+        if (const CompoundLiteralExpr *CLE =
+            dyn_cast<CompoundLiteralExpr>(ArgExpr))
+          if (const InitListExpr *ILE =
+              dyn_cast<InitListExpr>(CLE->getInitializer()))
+            ArgExpr = ILE->getInit(0);
+    }
+
+    bool Result;
+    if (ArgExpr->EvaluateAsBooleanCondition(Result, Context) && !Result)
       Diag(CallSiteLoc, diag::warn_null_arg) << ArgExpr->getSourceRange();
   }
 }