From c92d314009f477a7cb3ebd8dc9e320712ab7f3fd Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@pdimov.com>
Date: Sat, 1 Apr 2017 18:08:19 +0300
Subject: [PATCH] Implement mp_transform_if in terms of mp_transform

---
 include/boost/mp11/algorithm.hpp | 67 ++++++--------------------------
 test/mp_transform_if.cpp         | 10 +++++
 2 files changed, 22 insertions(+), 55 deletions(-)

diff --git a/include/boost/mp11/algorithm.hpp b/include/boost/mp11/algorithm.hpp
index f7d033d..503f5cd 100644
--- a/include/boost/mp11/algorithm.hpp
+++ b/include/boost/mp11/algorithm.hpp
@@ -105,65 +105,22 @@ template<template<class...> class F, template<class...> class L1, class... T1, t
 namespace detail
 {
 
-/*
 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...>;
+    // the stupid quote-unquote dance avoids "pack expansion used as argument for non-pack parameter of alias template"
+#if defined( BOOST_MSVC ) && BOOST_WORKAROUND( BOOST_MSVC, <= 1910 )
+
+    template<class... U> struct _f_ { using type = mp_eval_if<mp_not<mp_invoke<mp_quote<P>, U...>>, mp_first<mp_list<U...>>, mp_quote<F>::template fn, U...>; };
+    template<class... U> using _f = typename _f_<U...>::type;
+
+#else
+
+    template<class... U> using _f = mp_eval_if<mp_not<mp_invoke<mp_quote<P>, U...>>, mp_first<mp_list<U...>>, mp_quote<F>::template fn, U...>;
+
+#endif
+
     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...>>
-{
-#if defined( BOOST_MSVC ) && BOOST_WORKAROUND( BOOST_MSVC, <= 1910 )
-
-    template<class U> struct _f { using type = mp_eval_if<mp_not<P<U>>, U, F, U>; };
-    using type = L<typename _f<T>::type...>;
-
-#else
-
-    template<class U> using _f = mp_eval_if<mp_not<P<U>>, U, F, U>;
-    using type = L<_f<T>...>;
-
-#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
 
diff --git a/test/mp_transform_if.cpp b/test/mp_transform_if.cpp
index 89292dd..048909a 100644
--- a/test/mp_transform_if.cpp
+++ b/test/mp_transform_if.cpp
@@ -26,6 +26,8 @@ template<class T> using add_pointer = 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;
+template<class T1, class T2, class T3, class T4> using fourth = T4;
+template<class T1, class T2, class T3, class T4, class T5> using fifth = T5;
 
 int main()
 {
@@ -58,6 +60,14 @@ int main()
     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&>>));
 
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_transform_if<is_not_ref, fourth, L1, 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, fourth, L2, 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, fourth, L3, L3, L3, mp_iota<mp_size<L3>>>, std::pair<mp_size_t<0>, X2&>>));
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_transform_if<is_not_ref, fifth, L1, L1, 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, fifth, L2, L2, 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, fifth, L3, L3, L3, L3, mp_iota<mp_size<L3>>>, std::pair<mp_size_t<0>, X2&>>));
+
     //
 
     return boost::report_errors();