Description of problem:

There's a patch in the eigen3 package that installs eigens "cmake/FindEigen3.cmake" file to /usr/share/cmake/Modules, which is the
path for cmake's own modules (i.e. the ones documented in cmake-modules(7)).

The FindEigen3.cmake file in the eigen source tree is designed to be copied to consuming projects source trees if they want to use it. By installing the module globally you break the build of anyone doing "find_package(Eigen3 REQUIRED)" then linking to "Eigen3::Eigen" because now cmake will use the find module you installed instead of the config module that eigen3 provides as a built artifact (and is installed by eigen3-devel at /usr/share/eigen3)

Version-Release number of selected component (if applicable):
eigen3-3.3.7-3.fc31.src.rpm

How reproducible:

Very

Steps to Reproduce:

install eigen3-devel

Configure a project that looks like the following:

CMakeLists.txt
cmake_minimum_required(VERSION 3.16)
project(some_project LANGUAGES CXX)
find_package(Eigen3 REQUIRED)
add_executable(some_project some_project.cpp)
target_link_libraries(some_project PRIVATE Eigen3::Eigen)

Actual results:

Build failure

Expected results:

Build success

Additional info:


It looks like this patch (to install the find module) was adopted from debian,
so it's not impossible that this will have compatibility impacts, that said I think eigen's config file defines the same variables as findEigen, so who knows.