From 8e053269a8db29398c27aacacd3daf3e46e6c54d Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@pdimov.com>
Date: Sat, 18 Mar 2017 19:48:07 +0200
Subject: [PATCH] Make mp_transform_if variadic.

---
 include/boost/mp11/algorithm.hpp | 49 ++++++++++++++++++++++++++++++--
 test/mp_transform_if.cpp         | 18 +++++++++++-
 2 files changed, 63 insertions(+), 4 deletions(-)

diff --git a/include/boost/mp11/algorithm.hpp b/include/boost/mp11/algorithm.hpp
index 8930660..144f1f9 100644
--- a/include/boost/mp11/algorithm.hpp
+++ b/include/boost/mp11/algorithm.hpp
@@ -71,11 +71,20 @@ template<template<class...> class F, template<class...> class L1, class... T1, t
 
 template<template<class...> class F, class... L> using mp_transform = typename detail::mp_transform_impl<F, L...>::type;
 
-// mp_transform_if<P, F, L>
+// mp_transform_if<P, F, L...>
 namespace detail
 {
 
-template<template<class...> class P, template<class...> class F, class L> struct mp_transform_if_impl;
+/*
+template<template<class...> class P, template<class...> class F, class... L> struct mp_transform_if_impl
+{
+    // error: pack expansion used as argument for non-pack parameter of alias template
+    template<class... U> using _f = mp_eval_if<mp_not<P<U...>>, mp_first<mp_list<U...>>, F, U...>;
+    using type = mp_transform<_f, L...>;
+};
+*/
+
+template<template<class...> class P, template<class...> class F, class... L> struct mp_transform_if_impl;
 
 template<template<class...> class P, template<class...> class F, template<class...> class L, class... T> struct mp_transform_if_impl<P, F, L<T...>>
 {
@@ -92,9 +101,43 @@ template<template<class...> class P, template<class...> class F, template<class.
 #endif
 };
 
+template<template<class...> class P, template<class...> class F, template<class...> class L1, class... T1, template<class...> class L2, class... T2> struct mp_transform_if_impl<P, F, L1<T1...>, L2<T2...>>
+{
+    static_assert( sizeof...(T1) == sizeof...(T2), "The arguments of mp_transform_if should be of the same size" );
+
+#if defined( BOOST_MSVC ) && BOOST_WORKAROUND( BOOST_MSVC, <= 1910 )
+
+    template<class U1, class U2> struct _f { using type = mp_eval_if<mp_not<P<U1, U2>>, U1, F, U1, U2>; };
+    using type = L1<typename _f<T1, T2>::type...>;
+
+#else
+
+    template<class U1, class U2> using _f = mp_eval_if<mp_not<P<U1, U2>>, U1, F, U1, U2>;
+    using type = L1<_f<T1, T2>...>;
+
+#endif
+};
+
+template<template<class...> class P, template<class...> class F, template<class...> class L1, class... T1, template<class...> class L2, class... T2, template<class...> class L3, class... T3> struct mp_transform_if_impl<P, F, L1<T1...>, L2<T2...>, L3<T3...>>
+{
+    static_assert( sizeof...(T1) == sizeof...(T2) && sizeof...(T1) == sizeof...(T3), "The arguments of mp_transform_if should be of the same size" );
+
+#if defined( BOOST_MSVC ) && BOOST_WORKAROUND( BOOST_MSVC, <= 1910 )
+
+    template<class U1, class U2, class U3> struct _f { using type = mp_eval_if<mp_not<P<U1, U2, U3>>, U1, F, U1, U2, U3>; };
+    using type = L1<typename _f<T1, T2, T3>::type...>;
+
+#else
+
+    template<class U1, class U2, class U3> using _f = mp_eval_if<mp_not<P<U1, U2, U3>>, U1, F, U1, U2, U3>;
+    using type = L1<_f<T1, T2, T3>...>;
+
+#endif
+};
+
 } // namespace detail
 
-template<template<class...> class P, template<class...> class F, class L> using mp_transform_if = typename detail::mp_transform_if_impl<P, F, L>::type;
+template<template<class...> class P, template<class...> class F, class... L> using mp_transform_if = typename detail::mp_transform_if_impl<P, F, L...>::type;
 
 // mp_fill<L, V>
 namespace detail
diff --git a/test/mp_transform_if.cpp b/test/mp_transform_if.cpp
index 0151ac5..89292dd 100644
--- a/test/mp_transform_if.cpp
+++ b/test/mp_transform_if.cpp
@@ -23,12 +23,18 @@ struct X4 {};
 using boost::mp11::mp_not;
 
 template<class T> using add_pointer = T*;
-template<class T> using is_not_ref = mp_not<std::is_reference<T>>;
+template<class T, class...> using is_not_ref = mp_not<std::is_reference<T>>;
+template<class T1, class T2> using second = T2;
+template<class T1, class T2, class T3> using third = T3;
 
 int main()
 {
     using boost::mp11::mp_list;
     using boost::mp11::mp_transform_if;
+    using boost::mp11::mp_size_t;
+    using boost::mp11::mp_size;
+    using boost::mp11::mp_fill;
+    using boost::mp11::mp_iota;
 
     using L1 = mp_list<X1, X2&, X3 const, X4 const&>;
 
@@ -44,5 +50,15 @@ int main()
 
     //
 
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_transform_if<is_not_ref, second, L1, mp_fill<L1, void>>, mp_list<void, X2&, void, X4 const&>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_transform_if<is_not_ref, second, L2, mp_fill<L2, void>>, std::tuple<void, X2&, void, X4 const&>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_transform_if<is_not_ref, second, L3, mp_fill<L3, void>>, std::pair<void, X2&>>));
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_transform_if<is_not_ref, third, L1, L1, mp_iota<mp_size<L1>>>, mp_list<mp_size_t<0>, X2&, mp_size_t<2>, X4 const&>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_transform_if<is_not_ref, third, L2, L2, mp_iota<mp_size<L2>>>, std::tuple<mp_size_t<0>, X2&, mp_size_t<2>, X4 const&>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_transform_if<is_not_ref, third, L3, L3, mp_iota<mp_size<L3>>>, std::pair<mp_size_t<0>, X2&>>));
+
+    //
+
     return boost::report_errors();
 }