is(): made is() compile-time when possible

Author: filipsajdak

include/cpp2util.h

@@ -1629,13 +1629,13 @@ constexpr auto is( X&& ) {
 //  Types
 //
 template< typename C, typename X >
-auto is( X const& x ) -> bool {
+constexpr auto is( X const& x ) -> auto {
     if constexpr (
         std::is_same_v<C, X>
         || std::is_base_of_v<C, X>
     )
     {
-        return true;
+        return std::true_type{};
     }
     else if constexpr (
         std::is_polymorphic_v<C>
@@ -1660,12 +1660,12 @@ auto is( X const& x ) -> bool {
     )
     {
         if (x != nullptr) {
-            return is<std::remove_pointer_t<C>>(*x);
+            return bool{is<std::remove_pointer_t<C>>(*x)};
         }
         return false;
     }
     else {
-        return false;
+        return std::false_type{};
     }
 }