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I was previously thinking of adding a separate method:

• formula: add directory forbash-completion@2 completions #14270

But I wasn't happy on naming as version number doesn't make much sense given the path will likely remain for v3, v4, etc as it is based on XDG.

This PR is a different idea for DSL of using an argument, with default to keep using compatdir path so allbash_completion.install usage remains the same.

This is needed as we recently enabled Click Bash completions

• Should:click shell completions be enabled for Bash? #20188

Which are incompatible with system Bash

• https://github.com/orgs/Homebrew/discussions/6603

One scenario that this does not automatically support is Bash 4 withoutbash-completion@2 installed. For now, users will need to manually adjust their Bash config file to load these.

We could adjusthttps://docs.brew.sh/Shell-Completion#configuring-completions-in-bash but it will make the logic more complicated to detect which Bash version is running and add multiple directories to load from.

Easier response to this scenario is to tell users to installbash-completion@2.