Tidy Conclusion

Instant

Destroy target creature. You gain 1 life for each artifact you control.

2016-09-20:
The number of artifacts you control is counted only as Tidy Conclusion resolves, after the creature is destroyed. For example, if you destroy a Fairgrounds Warden that exiled your artifact creature, the artifact creature will return and then be counted when determining how many artifacts you control.
2016-09-20:
If the target creature becomes an illegal target before Tidy Conclusion resolves, the spell doesn't resolve. You don't gain any life.