Fix compilation issues related to Eigen 5

[jcarpent]

No description provided.

[jcarpent]

@Lucas-Haubert @jorisv The issue on the Ubuntu CI with the GeneralizedSelfAdjointEigenSolver is not related to this PR, and seems related either to an Eigen issue or to a test issue. @Lucas-Haubert Could you investigate it?

[jcarpent]

Fixes #586

[jorisv]

We can wait the conda-forge eigen 5 release so we will be able to reproduce the issue easily.

[traversaro]

We can wait the conda-forge eigen 5 release so we will be able to reproduce the issue easily.

This is fun, as I ended up here as I wanted to write in that PR "let's wait for downstream projects to start supporting Eigen 5 before upgrading the conda-forge package to Eigen", so that turned in a chicken and egg problems. Jokes apart, I guess this may take a few days, see conda-forge/eigen-feedstock#47 (comment) .

[nim65s]

I can switch the Nix CI to eigen5 if that help to break the deadlock

[jorisv]

@nim65s Thank, we were speaking about that with @ManifoldFR

[ManifoldFR]

Guilhem to the rescue 🙏🏼

[jorisv]

@traversaro I my point of view the new package should be released, then downstream project can add compatibility to it.

It's a new major version so if it break thing it's normal. People will provide patches. And if there is some bug on eigen side, they will (hopfully) release new patch release (:crossed_fingers:).
I don't really see the difference with a standard conda-forge migration (except the release has been messy and the ABI management you try to add in the conda-forge eigen package).

On CI side we have lockfile to protect us against that. But I know a lot of other projects doesn't have one, so I'm really speaking for myself.

[jorisv]

Wait ability to reproduce in CI

[traversaro]

@traversaro I my point of view the new package should be released, then downstream project can add compatibility to it.

Sure, I agree in theory, but eigen is a bit specific case for several reasons, including the fact that it is an header only package, so there is no usual migration process (at least until we will have the eigen-abi package, and even in that case the migration will only happen if you use eigen in the public headers, not if you have any transitive dependency that uses eigen, as it happens for "normal" libraries), that it has not being released for a long time, and is used really extensively in most C++ robotics libraries. Indeed my suggestion is just to wait for a few days, not forever for all downstream packages to be fixed (that will probably take months/years).