Flag to force the use of the latest allowed package

I am thinking about how to make dependency upgrades more automatic and more reliable (https://mobile.twitter.com/nomeata/status/1558859792919191552).

Imagine we have a tool or bot that creates tentatively PRs that extend the dependency ranges of your package `pkg`, e.g. from `foo <2.0` to `foo <2.1`. Ideally, if the CI for this change is green it should be ok to merge it (after also checking the changelog for semantic changes, of course).

But right now, a typical CI setup will not provide that: If `pkgs` also depends on `bar`, and all versions of `bar` depend on `foo <2.0`, then Cabal will continue to pick `foo-1`, the build will succeed, and the PR may be merged. But once `bar` allows `foo-2.0`, now Cabal will pick that, and an incompatibility might cause the build or the tests to break – but now on master or on Hackage.

When doing dependency bumps locally, I'd use `--constraint="foo>=2.0"` to avoid this. But what can we do in automatic CI?

I could imagine a flag `--constrain-latest-deps=pkgs` that instructs the Cabal solver to use, for every dependency of `pkgs`, exactly the highest known version that is within `pkgs`’s version bounds. I can then use this flag in CI, and know that my upper version bounds are actually realizable.

Would that be useful, doable and welcome?