From 11621ac2bda514e7c9403fe4929c135a1a2fb8dd Mon Sep 17 00:00:00 2001
From: Edward Catmur <edward.catmur@mavensecurities.com>
Date: Mon, 9 Aug 2021 11:28:25 +0100
Subject: [PATCH] Require implicit conversion for hana::is_convertible by
 static_cast

In C++20, static_cast can perform aggregate construction (__cpp_aggregate_paren_init). This breaks the example by allowing construction of Employee from its single direct base Person.

Change the maybe_static_cast control expression to require implicit conversion. Since this doesn't allow conversion to cv void, add a to_impl for cv void To.
---
 include/boost/hana/core/to.hpp | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/boost/hana/core/to.hpp b/include/boost/hana/core/to.hpp
index 1b54115162..eeb148d937 100644
--- a/include/boost/hana/core/to.hpp
+++ b/include/boost/hana/core/to.hpp
@@ -49,8 +49,8 @@ BOOST_HANA_NAMESPACE_BEGIN
         };
 
         template <typename To, typename From>
-        struct maybe_static_cast<To, From, decltype((void)
-            static_cast<To>(std::declval<From>())
+        struct maybe_static_cast<To, From, decltype(
+            std::declval<void(&)(To)>()(std::declval<From>())
         )> {
             template <typename X>
             static constexpr To apply(X&& x)
@@ -63,6 +63,13 @@ BOOST_HANA_NAMESPACE_BEGIN
         : convert_detail::maybe_static_cast<To, From>
     { };
 
+    template <typename To, typename From>
+    struct to_impl<To, From, when<std::is_void<To>::value>> {
+        template <typename X>
+        static constexpr X apply(X&&)
+        { return; }
+    };
+
     template <typename To>
     struct to_impl<To, To> : embedding<> {
         template <typename X>