Work around macOS clang variant issue., again.

include/bitcoin/node/define.hpp

@@ -88,10 +88,15 @@ typedef system::chain::block::cptr block_t;
 ////    system::chain::block::cptr block;
 ////} xblock_t;
 
-typedef std::conditional_t<
-    is_same_type<std::size_t, uint64_t>,
+// std::variant types must be distinct, and xcode size_t is neither uint32_t 
+// nor uint64_t, so this ensures we have the distinct set of necessary types.
+using event_value =
+    iif<is_same_type<std::size_t, uint64_t>,
         std::variant<uint32_t, size_t, block_t>,
-        std::variant<uint64_t, size_t, block_t>> event_value;
+        iif<is_same_type<std::size_t, uint32_t>,
+            std::variant<uint64_t, size_t, block_t>,
+                std::variant<uint64_t, uint32_t, size_t, block_t>>>;
+
 typedef network::desubscriber<object_key, chase, event_value> event_subscriber;
 typedef event_subscriber::handler event_notifier;
 typedef event_subscriber::completer event_completer;