Imported Upstream version 1.67.0upstream/1.67.0

1 files changed, 21 insertions, 19 deletions

diff --git a/boost/hana/core/to.hpp b/boost/hana/core/to.hpp
index 6a6a115b3a..1b54115162 100644
--- a/boost/hana/core/to.hpp
+++ b/boost/hana/core/to.hpp
@@ -39,27 +39,29 @@ BOOST_HANA_NAMESPACE_BEGIN
     namespace convert_detail {
         struct no_conversion { };
 
-        template <typename ...>
-        struct is_valid { static constexpr bool value = true; };
-    }
+        template <typename To, typename From, typename = void>
+        struct maybe_static_cast : no_conversion {
+            template <typename X>
+            static constexpr auto apply(X const&) {
+                static_assert(detail::wrong<to_impl<To, From>, X>{},
+                "no conversion is available between the provided types");
+            }
+        };
 
-    template <typename To, typename From, bool condition>
-    struct to_impl<To, From, when<condition>> : convert_detail::no_conversion {
-        template <typename X>
-        static constexpr auto apply(X const&) {
-            static_assert(detail::wrong<to_impl<To, From>, X>{},
-            "no conversion is available between the provided types");
-        }
-    };
+        template <typename To, typename From>
+        struct maybe_static_cast<To, From, decltype((void)
+            static_cast<To>(std::declval<From>())
+        )> {
+            template <typename X>
+            static constexpr To apply(X&& x)
+            { return static_cast<To>(static_cast<X&&>(x)); }
+        };
+    } // end namespace convert_detail
 
-    template <typename To, typename From>
-    struct to_impl<To, From, when<convert_detail::is_valid<
-        decltype(static_cast<To>(std::declval<From>()))
-    >::value>> {
-        template <typename X>
-        static constexpr To apply(X&& x)
-        { return static_cast<To>(static_cast<X&&>(x)); }
-    };
+    template <typename To, typename From, bool condition>
+    struct to_impl<To, From, when<condition>>
+        : convert_detail::maybe_static_cast<To, From>
+    { };
 
     template <typename To>
     struct to_impl<To, To> : embedding<> {