From afd396692041044e9d8bc5c8b80444ede64c6e4b Mon Sep 17 00:00:00 2001
From: Kris Jusiak <kris@jusiak.net>
Date: Sat, 12 Jun 2021 17:42:23 +0000
Subject: [PATCH] :new: Add `mp_quote_expr` to quote with lambda expressions

Problem:
- There is no easy way to quote metafunction directly with lambda expressions.

Solution:
- Add mp_quote_expr.

Note:
- mp_quote_expr requires C++20.
---
 doc/mp11/utility.adoc                | 34 ++++++++++++++
 include/boost/mp11/detail/config.hpp | 12 +++++
 include/boost/mp11/utility.hpp       |  8 ++++
 test/Jamfile                         |  1 +
 test/mp_quote_expr.cpp               | 69 ++++++++++++++++++++++++++++
 5 files changed, 124 insertions(+)
 create mode 100644 test/mp_quote_expr.cpp

diff --git a/doc/mp11/utility.adoc b/doc/mp11/utility.adoc
index d5bb6220..a9ad9481 100644
--- a/doc/mp11/utility.adoc
+++ b/doc/mp11/utility.adoc
@@ -246,6 +246,40 @@ using R1 = mp_transform_q<mp_quote_trait<std::add_pointer>, L1>;
   // mp_list<int*, void*, float*>
 ```
 
+## mp_quote_expr<Expr>
+
+    template<auto Expr>
+    struct mp_quote_expr {
+        template<class... Ts>
+        using fn = decltype(Expr(std::declval<Ts&>()...));
+    };
+
+`mp_quote_expr<Expr>` transforms the template `Expr` into a _quoted metafunction_, a type with a nested template `fn` such that `fn<T...>` returns `Expr(std::declval<Ts&>()...)`.
+
+.Using mp_quote_expr to wrap types depending on whether they have a `value` or not
+```
+template<class> struct with_value {};
+template<class> struct without_value {};
+
+struct foo { int value{}; };
+struct bar { };
+
+using L1 = mp_list<foo, bar>;
+using R1 = mp_transform_q<
+    mp_quote_expr<[]<class T>(T t) {
+      if constexpr (requires { t.value; }) {
+        return with_value<T>{};
+      } else {
+        return without_value<T>{};
+      }
+    }>,
+    L1
+  >;
+  // mp_list<with_value<foo>, without_value<bar>>
+```
+
+> `mp_quote_trait<F>` requires C++20 support
+
 ## mp_invoke_q<Q, T...>
 
     template<class Q, class... T> using mp_invoke_q = typename Q::template fn<T...>;
diff --git a/include/boost/mp11/detail/config.hpp b/include/boost/mp11/detail/config.hpp
index 0a9ded50..b6c1f31b 100644
--- a/include/boost/mp11/detail/config.hpp
+++ b/include/boost/mp11/detail/config.hpp
@@ -113,6 +113,18 @@
 #  define BOOST_MP11_HAS_FOLD_EXPRESSIONS
 #endif
 
+// BOOST_MP11_HAS_NONTYPE_TEMPLATE_ARGS
+
+#if !defined(BOOST_MP11_HAS_NONTYPE_TEMPLATE_ARGS) && defined(__cpp_nontype_template_args) && __cpp_nontype_template_args >= 201911
+#  define BOOST_MP11_HAS_NONTYPE_TEMPLATE_ARGS
+#endif
+
+// BOOST_MP11_HAS_NONTYPE_TEMPLATE_PARAMETER_AUTO
+
+#if !defined(BOOST_MP11_HAS_NONTYPE_TEMPLATE_PARAMETER_AUTO) && defined(__cpp_nontype_template_parameter_auto) && __cpp_nontype_template_parameter_auto >= 201606
+#  define BOOST_MP11_HAS_NONTYPE_TEMPLATE_PARAMETER_AUTO
+#endif
+
 // BOOST_MP11_HAS_TYPE_PACK_ELEMENT
 
 #if defined(__has_builtin)
diff --git a/include/boost/mp11/utility.hpp b/include/boost/mp11/utility.hpp
index 533009aa..0447dfba 100644
--- a/include/boost/mp11/utility.hpp
+++ b/include/boost/mp11/utility.hpp
@@ -251,6 +251,14 @@ template<class... Q> struct mp_compose_q
     template<class T> using fn = mp_fold<mp_list<Q...>, T, detail::mp_reverse_invoke_q>;
 };
 
+#if defined(BOOST_MP11_HAS_NONTYPE_TEMPLATE_ARGS) && defined(BOOST_MP11_HAS_NONTYPE_TEMPLATE_PARAMETER_AUTO)
+template<auto Expr>
+struct mp_quote_expr {
+    template<class... Ts>
+    using fn = decltype(Expr(std::declval<Ts&>()...));
+};
+#endif
+
 } // namespace mp11
 } // namespace boost
 
diff --git a/test/Jamfile b/test/Jamfile
index 0a1be298..b660529a 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -142,6 +142,7 @@ run mp_defer.cpp ;
 run mp_quote.cpp ;
 run mp_invoke_q.cpp ;
 run mp_invoke_q_sf.cpp ;
+run mp_quote_expr.cpp ;
 run mp_quote_trait.cpp ;
 run mp_cond.cpp ;
 run mp_cond_sf.cpp ;
diff --git a/test/mp_quote_expr.cpp b/test/mp_quote_expr.cpp
new file mode 100644
index 00000000..4c30240d
--- /dev/null
+++ b/test/mp_quote_expr.cpp
@@ -0,0 +1,69 @@
+
+//  Copyright 2021 Kris Jusiak.
+//
+// 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/mp11/algorithm.hpp>
+#include <boost/mp11/list.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+
+int main()
+{
+#if defined(BOOST_MP11_HAS_NONTYPE_TEMPLATE_ARGS) && defined(BOOST_MP11_HAS_NONTYPE_TEMPLATE_PARAMETER_AUTO)
+    using boost::mp11::mp_list;
+    using boost::mp11::mp_transform_q;
+    using boost::mp11::mp_quote_expr;
+
+    constexpr auto expr = [](auto t) { return t; };
+
+    {
+      BOOST_TEST_TRAIT_TRUE((std::is_same<
+        mp_list<>,
+        mp_transform_q<
+          mp_quote_expr<expr>,
+          mp_list<>
+        >
+      >));
+    }
+
+    {
+      BOOST_TEST_TRAIT_TRUE((std::is_same<
+        mp_list<int>,
+        mp_transform_q<
+          mp_quote_expr<expr>,
+          mp_list<int>
+        >
+      >));
+    }
+
+    struct a{};
+    struct b{};
+
+    {
+      BOOST_TEST_TRAIT_TRUE((std::is_same<
+        mp_list<a, b>,
+        mp_transform_q<
+          mp_quote_expr<expr>,
+          mp_list<a, b>
+        >
+      >));
+    }
+
+    {
+      constexpr auto to_ptr = [](auto& t) { return &t; };
+
+      BOOST_TEST_TRAIT_TRUE((std::is_same<
+        mp_list<a*, b*>,
+        mp_transform_q<
+          mp_quote_expr<to_ptr>,
+          mp_list<a, b>
+        >
+      >));
+    }
+#endif
+    return boost::report_errors();
+}