How to find the Eigen in the same cmake project?

[ziyuang]

In CMake\HighFiveTargetDeps.cmake the relevant command for including Eigen is target_include_directories(libdeps SYSTEM INTERFACE ${EIGEN3_INCLUDE_DIRS}). This seems to prevent including Eigen in the same project, as doing so by set(EIGEN3_INCLUDE_DIRS ${eigen_SOURCE_DIR}) after FetchContenting Eigen will raise the complaint

 Target "libdeps" INTERFACE_INCLUDE_DIRECTORIES property contains path:

    "<project directory>/build/_deps/eigen-src"

  which is prefixed in the build directory.

[1uc]

A think a similar issue is encountered in #766. A solution would be to add the HighFive sources to your project without performing the add_subdirectory. Then you simply reproduce the required logic yourself. See,
https://github.com/BlueBrain/libsonata/blob/f26db3226ce40a02373a9794956f23f0013de8b3/extlib/CMakeLists.txt#L2

for one example for how this could be done.