When your pull request breaks an external project we test in our CI and you have prepared a branch with the fix, you can add an "overlay" to your pull request to test it with the adapted version of the external project.
An overlay is a file which defines where to look for the patched
version so that testing is possible.
The name of your overlay file should start with a five-digit pull request
number, followed by a dash, anything (for instance your GitHub nickname
and the branch name), then a .sh extension ([0-9]{5}-[a-zA-Z0-9-_]+.sh).
This file must contain one or more invocation of the overlay function:
overlay <project> <giturl> <ref> <prnumber> [<prbranch>]
Each call creates an overlay for project using a given giturl and
ref which is active for prnumber or prbranch (prbranch defaults
to ref).
Example of an overlay for the project elpi that uses the branch noinstance
from the fork of SkySkimmer and is active for pull request 13128
overlay elpi https://github.com/SkySkimmer/coq-elpi noinstance 13128
Such a file can be created automatically using the scripts
create_overlays.sh.
See also the list of projects for which one can write an overlay in
the file ci-basic-overlay.sh.
We suggest you use the convention of identical branch names for the
Coq branch and the CI project branch used in the overlay. For example,
if your Coq PR is coming from the branch more_efficient_tc, and that
breaks ltac2, we suggest you create a ltac2 overlay with a branch
named more_efficient_tc.