tile/base/future/boxed.h

@@ -86,7 +86,7 @@ private:
   }
 
 private:
-  std::variant<std::monostate, value_type> holding_;
+  std::variant<mpark::monostate, value_type> holding_;
 };
 
 static_assert(std::is_void<decltype(std::declval<Boxed<>>().Get())>::value, "");