From 4515938bbd4e32de758adcbe52157540559c8347 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Sat, 25 Jul 2020 01:56:00 +0300
Subject: [PATCH] Work around std::tuple MSVC issue with mp_map_find (fixes
 #52)

---
 include/boost/mp11/detail/mp_map_find.hpp | 52 +++++++++++++++++++++--
 1 file changed, 48 insertions(+), 4 deletions(-)

diff --git a/include/boost/mp11/detail/mp_map_find.hpp b/include/boost/mp11/detail/mp_map_find.hpp
index ec0fabd..5ba58a4 100644
--- a/include/boost/mp11/detail/mp_map_find.hpp
+++ b/include/boost/mp11/detail/mp_map_find.hpp
@@ -9,6 +9,17 @@
 //  http://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/mp11/utility.hpp>
+#include <boost/mp11/detail/config.hpp>
+
+#if BOOST_MP11_WORKAROUND( BOOST_MP11_MSVC, < 1930 )
+
+// not exactly good practice, but...
+namespace std
+{
+    template<class... _Types> class tuple;
+}
+
+#endif
 
 namespace boost
 {
@@ -19,18 +30,51 @@ namespace mp11
 namespace detail
 {
 
+#if !BOOST_MP11_WORKAROUND( BOOST_MP11_MSVC, < 1930 )
+
+template<class T> using mpmf_wrap = mp_identity<T>;
+template<class T> using mpmf_unwrap = typename T::type;
+
+#else
+
+template<class... T> struct mpmf_tuple {};
+
+template<class T> struct mpmf_wrap_impl
+{
+    using type = mp_identity<T>;
+};
+
+template<class... T> struct mpmf_wrap_impl< std::tuple<T...> >
+{
+    using type = mp_identity< mpmf_tuple<T...> >;
+};
+
+template<class T> using mpmf_wrap = typename mpmf_wrap_impl<T>::type;
+
+template<class T> struct mpmf_unwrap_impl
+{
+    using type = typename T::type;
+};
+
+template<class... T> struct mpmf_unwrap_impl< mp_identity< mpmf_tuple<T...> > >
+{
+    using type = std::tuple<T...>;
+};
+
+template<class T> using mpmf_unwrap = typename mpmf_unwrap_impl<T>::type;
+
+#endif // #if !BOOST_MP11_WORKAROUND( BOOST_MP11_MSVC, < 1930 )
+
 template<class M, class K> struct mp_map_find_impl;
 
 template<template<class...> class M, class... T, class K> struct mp_map_find_impl<M<T...>, K>
 {
-    using U = mp_inherit<mp_identity<T>...>;
+    using U = mp_inherit<mpmf_wrap<T>...>;
 
     template<template<class...> class L, class... U> static mp_identity<L<K, U...>> f( mp_identity<L<K, U...>>* );
     static mp_identity<void> f( ... );
 
-    using V = decltype( f((U*)0) );
-
-    using type = typename V::type;
+    using type = mpmf_unwrap< decltype( f((U*)0) ) >;
 };
 
 } // namespace detail