Really needed for customizing web applications (even home made) and getting a good IDE integration even for war projects (take a look at STS-1314 discussion).
Maybe this could also benefit from GRADLE-1014?

I hope this issue should not remain a minor priority RFE.
I think here the focus doesn't lie as much in having any way to produce a war (we can still obtain it in several ways thanks to gradle flexibility) but rather in composing war artifacts based on dependencies between wars, with proper management for provided deps transitivity.
In other words a war project by convention could get transient deps and web contents from other war projects/artifacts it depends upon.
It would be great also having a way to obtain war overlay for gradle projects that enforces complete IDE integration i.e. a declarative approach that lets the tooling api expose enough info about merged resources in order to support the ide on reflecting the overlay as needed (we use eclipse/STS).
I've opened a dedicated thread on the user mailing list.

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From my perspective you should be able to achieve the same behavior with methods already exposed by AbstractCopyTask which effectively is being used by the War task. You can easily merge existing WARs into other WARs, filter files as you merge them and exclude any files you need to filter. I am going to mark this issue as "Won't fix". Please open an issue on GitHub if you feel like your use case isn't fulfilled.