Make the E parameter of mp_if optional

test/Jamfile.v2

@@ -75,6 +75,7 @@ run integral.cpp : : : $(REQ) ;
 run mp_identity.cpp : : : $(REQ) ;
 run mp_inherit.cpp : : : $(REQ) ;
 run mp_if.cpp : : : $(REQ) ;
+run mp_if_sf.cpp : : : $(REQ) ;
 run mp_eval_if.cpp : : : $(REQ) ;
 run mp_valid.cpp : : : $(REQ) ;
 run mp_defer.cpp : : : $(REQ) ;

test/mp_if_sf.cpp

@@ -0,0 +1,24 @@
+
+//  Copyright 2017 Peter Dimov.
+//
+// Distributed under the Boost Software License, Version 1.0.
+//
+// See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt
+
+
+#include <boost/mp11/utility.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <type_traits>
+
+int main()
+{
+    using boost::mp11::mp_if;
+    using boost::mp11::mp_valid;
+
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_if, std::false_type, void>));
+    BOOST_TEST_TRAIT_TRUE((mp_valid<mp_if, std::false_type, void, void>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_if, std::false_type, void, void, void>));
+
+    return boost::report_errors();
+}