diff --git a/scripts/qapi-types.py b/scripts/qapi-types.py
index 40e9f79b63..f9fcf150a4 100644
--- a/scripts/qapi-types.py
+++ b/scripts/qapi-types.py
@@ -98,6 +98,19 @@ struct %(c_name)s {
     return ret
 
 
+def gen_upcast(name, base):
+    # C makes const-correctness ugly.  We have to cast away const to let
+    # this function work for both const and non-const obj.
+    return mcgen('''
+
+static inline %(base)s *qapi_%(c_name)s_base(const %(c_name)s *obj)
+{
+    return (%(base)s *)obj;
+}
+''',
+                 c_name=c_name(name), base=base.c_name())
+
+
 def gen_alternate_qtypes_decl(name):
     return mcgen('''
 
@@ -267,6 +280,9 @@ class QAPISchemaGenTypeVisitor(QAPISchemaVisitor):
         if variants:
             assert not members      # not implemented
             self.decl += gen_union(name, base, variants)
+            # TODO Use gen_upcast on structs too, once they have sane layout
+            if base:
+                self.decl += gen_upcast(name, base)
         else:
             self.decl += gen_struct(name, base, members)
         self._gen_type_cleanup(name)
diff --git a/tests/test-qmp-input-visitor.c b/tests/test-qmp-input-visitor.c
index 8941963c8d..da21709714 100644
--- a/tests/test-qmp-input-visitor.c
+++ b/tests/test-qmp-input-visitor.c
@@ -347,6 +347,7 @@ static void test_visitor_in_union_flat(TestInputVisitorData *data,
     Visitor *v;
     Error *err = NULL;
     UserDefFlatUnion *tmp;
+    UserDefUnionBase *base;
 
     v = visitor_input_test_init(data,
                                 "{ 'enum1': 'value1', "
@@ -360,6 +361,10 @@ static void test_visitor_in_union_flat(TestInputVisitorData *data,
     g_assert_cmpstr(tmp->string, ==, "str");
     g_assert_cmpint(tmp->integer, ==, 41);
     g_assert_cmpint(tmp->value1->boolean, ==, true);
+
+    base = qapi_UserDefFlatUnion_base(tmp);
+    g_assert(&base->enum1 == &tmp->enum1);
+
     qapi_free_UserDefFlatUnion(tmp);
 }