Eigen3 makes use of the cmake user package registry through the cmake command "export(PACKAGE ...)". This command registers the Eigen3Config.cmake from *the build tree*. However, the relative paths currently used in this config file are designed to be correct within *the installation tree structure*. This results in CMake finding Eigen3 form downstream packages, but use incorrect include paths.
Note, that the behaviour was explicitly changed to relative paths in #1347.
Some bits of discussions there: https://bitbucket.org/eigen/eigen/commits/52399ea56fa9e936bd936e47c8a356265ecbfa92#LCMakeLists.txtT545
I will try to submit a PR on bitbucket to address this bug. Here is what I said in an email to firstname.lastname@example.org:
I believe the issue is that the build directory has an Eigen3Config.cmake file in it; the configured file in the build directory should be named something else so that CMake does not pick it up. For example, in my project, we name the configured build-directory file “OpenSimConfigToInstall.cmake”: https://github.com/opensim-org/opensim-core/blob/master/cmake/CMakeLists.txt#L12. This resolved the issue for us.
I recently ran into this as well, when a library used
find_package(Eigen3 3.0.5 QUIET CONFIG)
and accidentally picked up a local Eigen3Config.cmake from a version which had some experimental (not yet working) changes.
In my eyes this PR is good:
But I'm not really a CMake expert, so I have no idea, whether one should or should not do:
export (PACKAGE Eigen3)