Workaround for #3140 Compilation error using newer Eigen library.

[arianepaola]

This pull request is a workaround for #3140 causing a compilation error, when using Eigen 3.3 versions.
It adds an additional check to verify that the Eigen version is between 3.1.0 and 3.2.5. (excluding >= 3.2.6)

[vigsterkr]

@arianepaola good, but would be better if we could actually solve this by actually have a solution for 3.3+ :) because now as far as i understand this will just not use eigen 3.3+, or?

[karlnapf]

Can you use the CMake function VERSION_LESS
See e.g. examples/meta/cpp/CMakeLists.txt
It is a bit cleaner

[karlnapf]

I agree with @vigsterkr on fixing this somehow rather than excluding new eigen versions.
However, I am fine to merge this as a temp. solution as we can just bundle eigen if the system version is too recent. I leave it to @vigsterkr to decide

[vigsterkr]

@karlnapf no temp merge. i hate temp merges 💃

[vigsterkr]

i mean i really appreciate the effort here! i do! but let's try to fix it so that 3.3+ can play as well. should not be that hard

[vigsterkr]

btw i should have written after the 💃 caramba!

[karlnapf]

hahahah ;)

[karlnapf]

I would merge this one. Requiring a stable eigen version (and bundling if not available) is fine in my eyes. Once the new eigen is out, we can change that

[vigsterkr]

CARAMBA! 🚯
be more patient :) i'll add patches here and then let's merge it plz.

[arianepaola]

@vigsterkr @karlnapf Thanks for the comments, they make sense.
But as an example Ubuntu 16.04 will be released this month and it is usual to release with beta packages. Unfortunately the Eigen 3.3 beta also uses a 3.2.x version numbering.

I have modified my pull request to take out the version numbers from the source file and to rely only on one preprocessor definition. This moves the minimum Eigen version to a preprocessor definition and also resolves a mismatch between the source files (e.g. at least 3.1.0) and the CMake required Eigen 3.1.2.
Now EIGEN_VERSION_DEPENDABLE surrounds the previous code block.

It is not a good idea to force the download of earlier Eigen versions, such as the defined 3.2.8 in cmake/external/Eigen3.cmake as it will hide bugs such as #3140 and will hinder testing shogun on newer versions.

The modified pull request will still trigger the error in #3140 and provide a temporary work around for #3142.

After providing a final work around for #3142, the CMake IF clause can be modified to just use at least Eigen 3.1.2.

Thanks, Ariane