Remove mp_defer_if_valid; add mp_identity_t.

include/boost/mp11/utility.hpp

@@ -19,6 +19,9 @@ template<class T> struct mp_identity
     using type = T;
 };
 
+// mp_identity_t
+template<class T> using mp_identity_t = T;
+
 // mp_inherit
 template<class... T> struct mp_inherit: T... {};
 
@@ -82,13 +85,21 @@ template<template<class...> class F, class... T> struct mp_valid_impl
 template<template<class...> class F, class... T> using mp_valid = typename detail::mp_valid_impl<F, T...>::type;
 
 // mp_defer
-template<template<class...> class F, class... T> struct mp_defer
+namespace detail
+{
+
+template<template<class...> class F, class... T> struct mp_defer_impl
 {
     using type = F<T...>;
 };
 
-// mp_defer_if_valid
-template<template<class...> class F, class... T> using mp_defer_if_valid = mp_if<mp_valid<F, T...>, mp_defer<F, T...>, mp_inherit<>>;
+struct mp_no_type
+{
+};
+
+} // namespace detail
+
+template<template<class...> class F, class... T> using mp_defer = mp_if<mp_valid<F, T...>, detail::mp_defer_impl<F, T...>, detail::mp_no_type>;
 
 } // namespace boost
 

test/Jamfile.v2

@@ -66,7 +66,6 @@ run mp_if.cpp : : : $(REQ) ;
 run mp_eval_if.cpp : : : $(REQ) ;
 run mp_valid.cpp : : : $(REQ) ;
 run mp_defer.cpp : : : $(REQ) ;
-run mp_defer_if_valid.cpp : : : $(REQ) ;
 
 # integer_sequence
 run integer_sequence.cpp : : : $(REQ) ;

test/mp_defer.cpp

@@ -8,19 +8,35 @@
 
 
 #include <boost/mp11/utility.hpp>
+#include <boost/mp11/integral.hpp>
 #include <boost/core/lightweight_test_trait.hpp>
 #include <type_traits>
 
-template<class T> using add_pointer = T*;
+template<class T> struct has_type
+{
+    template<class U> static boost::mp_true f( boost::mp_identity<typename U::type>* );
+    template<class U> static boost::mp_false f( ... );
+
+    using type = decltype( f<T>(0) );
+
+    static const auto value = type::value;
+};
 
 using boost::mp_defer;
 
-template<class T> using add_pointer_impl = mp_defer<add_pointer, T>;
+template<class T> using add_pointer = T*;
+template<class... T> using add_pointer_impl = mp_defer<add_pointer, T...>;
 
 int main()
 {
+    BOOST_TEST_TRAIT_TRUE((has_type<add_pointer_impl<void>>));
     BOOST_TEST_TRAIT_TRUE((std::is_same<add_pointer_impl<void>::type, void*>));
+
+    BOOST_TEST_TRAIT_TRUE((has_type<add_pointer_impl<int>>));
     BOOST_TEST_TRAIT_TRUE((std::is_same<add_pointer_impl<int>::type, int*>));
 
+    BOOST_TEST_TRAIT_FALSE((has_type<add_pointer_impl<>>));
+    BOOST_TEST_TRAIT_FALSE((has_type<add_pointer_impl<void, void>>));
+
     return boost::report_errors();
 }

test/mp_defer_if_valid.cpp

@@ -1,29 +0,0 @@
-
-//  Copyright 2015 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>
-
-using boost::mp_defer_if_valid;
-using boost::mp_identity;
-
-template<class... T> using mp_identity_2 = typename mp_defer_if_valid<mp_identity, T...>::type;
-
-int main()
-{
-    using boost::mp_valid;
-
-    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_identity_2>));
-    BOOST_TEST_TRAIT_TRUE((mp_valid<mp_identity_2, void>));
-    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_identity_2<void>, mp_identity<void>>));
-    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_identity_2, void, void>));
-
-    return boost::report_errors();
-}

test/mp_identity.cpp

@@ -22,5 +22,12 @@ int main()
     BOOST_TEST_TRAIT_TRUE((std::is_same<mp_identity<int const[]>::type, int const[]>));
     BOOST_TEST_TRAIT_TRUE((std::is_same<mp_identity<X>::type, X>));
 
+    using boost::mp_identity_t;
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_identity_t<void const volatile>, void const volatile>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_identity_t<void()>, void()>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_identity_t<int const[]>, int const[]>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_identity_t<X>, X>));
+
     return boost::report_errors();
 }