🆕 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

@@ -246,6 +246,35 @@ 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 find out struct with a value member
+```
+struct foo { int value{}; };
+struct bar { };
+
+using L1 = mp_list<int, foo, bar>;
+using R1 = mp_transform_q<
+    mp_quote_expr<[](auto t) {
+      if constexpr (requires { t.value; }) {
+        return t;
+      }
+    }>,
+    L1
+  >;
+  // mp_list<void, foo, void>
+```
+
+> `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...>;

include/boost/mp11/detail/config.hpp

@@ -113,6 +113,12 @@
 #  define BOOST_MP11_HAS_FOLD_EXPRESSIONS
 #endif
 
+// BOOST_MP11_HAS_NONTYPE_TEMPLATE
+
+#if !defined(BOOST_MP11_HAS_NONTYPE_TEMPLATE) && defined(__cpp_nontype_template_args) && defined(__cpp_nontype_template_parameter_auto) && __cplusplus >= 202002L && !BOOST_MP11_WORKAROUND( BOOST_MP11_CLANG, < 120 )
+#  define BOOST_MP11_HAS_NONTYPE_TEMPLATE
+#endif
+
 // BOOST_MP11_HAS_TYPE_PACK_ELEMENT
 
 #if defined(__has_builtin)

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)
+template<auto Expr>
+struct mp_quote_expr {
+    template<class... Ts>
+    using fn = decltype(Expr(std::declval<Ts&>()...));
+};
+#endif
+
 } // namespace mp11
 } // namespace boost
 

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 ;

test/mp_quote_expr.cpp

@@ -0,0 +1,70 @@
+
+//  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)
+    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();
+}