From 73626ba3bc992f11acfb5c9c887128f66e5b2013 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Tue, 16 May 2023 19:13:56 +0300
Subject: [PATCH] Add value list support to mp_at. Refs #53.

---
 include/boost/mp11/algorithm.hpp | 11 +++++-
 test/Jamfile                     |  1 +
 test/mp_at_2.cpp                 | 64 ++++++++++++++++++++++++++++++++
 3 files changed, 75 insertions(+), 1 deletion(-)
 create mode 100644 test/mp_at_2.cpp

diff --git a/include/boost/mp11/algorithm.hpp b/include/boost/mp11/algorithm.hpp
index e73e7fe..82a7aa9 100644
--- a/include/boost/mp11/algorithm.hpp
+++ b/include/boost/mp11/algorithm.hpp
@@ -352,11 +352,20 @@ template<template<class...> class L, class... T, std::size_t I> struct mp_at_c_i
     using type = __type_pack_element<I, T...>;
 };
 
+#if defined(BOOST_MP11_HAS_TEMPLATE_AUTO)
+
+template<template<auto...> class L, auto... A, std::size_t I> struct mp_at_c_impl<L<A...>, I>
+{
+    using type = __type_pack_element<I, mp_value<A>...>;
+};
+
+#endif
+
 #else
 
 template<class L, std::size_t I> struct mp_at_c_impl
 {
-    using _map = mp_transform<mp_list, mp_iota<mp_size<L> >, L>;
+    using _map = mp_transform<mp_list, mp_iota<mp_size<L> >, mp_rename<L, mp_list>>;
     using type = mp_second<mp_map_find<_map, mp_size_t<I> > >;
 };
 
diff --git a/test/Jamfile b/test/Jamfile
index 009cf72..886a31c 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -94,6 +94,7 @@ run mp_drop.cpp ;
 run mp_iota.cpp ;
 run mp_iota_2.cpp ;
 run mp_at.cpp ;
+run mp_at_2.cpp ;
 run mp_at_sf.cpp : : : <toolset>gcc-4.7:<warnings>all ;
 run mp_take.cpp ;
 run mp_replace.cpp ;
diff --git a/test/mp_at_2.cpp b/test/mp_at_2.cpp
new file mode 100644
index 0000000..c111d3a
--- /dev/null
+++ b/test/mp_at_2.cpp
@@ -0,0 +1,64 @@
+// Copyright 2023 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/mp11/algorithm.hpp>
+#include <boost/mp11/integral.hpp>
+
+#if !defined(BOOST_MP11_HAS_TEMPLATE_AUTO)
+
+#pragma message("Test skipped because BOOST_MP11_HAS_TEMPLATE_AUTO is not defined")
+int main() {}
+
+#else
+
+#include <boost/core/lightweight_test_trait.hpp>
+
+template<auto... A> struct V1 {};
+template<int... I> struct V2 {};
+
+int main()
+{
+    using boost::mp11::mp_at;
+    using boost::mp11::mp_at_c;
+    using boost::mp11::mp_false;
+    using boost::mp11::mp_true;
+    using boost::mp11::mp_int;
+    using boost::mp11::mp_size_t;
+
+    {
+        using L1 = V1<false, 0, true, 1, std::size_t(2)>;
+
+        BOOST_TEST_TRAIT_SAME(mp_at_c<L1, 0>, mp_false);
+        BOOST_TEST_TRAIT_SAME(mp_at_c<L1, 1>, mp_int<0>);
+        BOOST_TEST_TRAIT_SAME(mp_at_c<L1, 2>, mp_true);
+        BOOST_TEST_TRAIT_SAME(mp_at_c<L1, 3>, mp_int<1>);
+        BOOST_TEST_TRAIT_SAME(mp_at_c<L1, 4>, mp_size_t<2>);
+
+        BOOST_TEST_TRAIT_SAME(mp_at<L1, mp_size_t<0>>, mp_false);
+        BOOST_TEST_TRAIT_SAME(mp_at<L1, mp_size_t<1>>, mp_int<0>);
+        BOOST_TEST_TRAIT_SAME(mp_at<L1, mp_size_t<2>>, mp_true);
+        BOOST_TEST_TRAIT_SAME(mp_at<L1, mp_size_t<3>>, mp_int<1>);
+        BOOST_TEST_TRAIT_SAME(mp_at<L1, mp_size_t<4>>, mp_size_t<2>);
+    }
+
+    {
+        using L1 = V2<1, 2, 3, 4, 5>;
+
+        BOOST_TEST_TRAIT_SAME(mp_at_c<L1, 0>, mp_int<1>);
+        BOOST_TEST_TRAIT_SAME(mp_at_c<L1, 1>, mp_int<2>);
+        BOOST_TEST_TRAIT_SAME(mp_at_c<L1, 2>, mp_int<3>);
+        BOOST_TEST_TRAIT_SAME(mp_at_c<L1, 3>, mp_int<4>);
+        BOOST_TEST_TRAIT_SAME(mp_at_c<L1, 4>, mp_int<5>);
+
+        BOOST_TEST_TRAIT_SAME(mp_at<L1, mp_size_t<0>>, mp_int<1>);
+        BOOST_TEST_TRAIT_SAME(mp_at<L1, mp_size_t<1>>, mp_int<2>);
+        BOOST_TEST_TRAIT_SAME(mp_at<L1, mp_size_t<2>>, mp_int<3>);
+        BOOST_TEST_TRAIT_SAME(mp_at<L1, mp_size_t<3>>, mp_int<4>);
+        BOOST_TEST_TRAIT_SAME(mp_at<L1, mp_size_t<4>>, mp_int<5>);
+    }
+
+    return boost::report_errors();
+}
+
+#endif