[2023] inconsistent eigen package

TL;DR
Setup
Problem
Details
Workaround

TL;DR

Eigen cmake support seems to be inconsistent

Setup

library xo-kalmanfilter depends on eigen
library xo-pykalmanfilter depends on xo-kalmanfilter
the project xo-umbrella2 incorporates both codebases (along with others) into a single source tree (using git subrepo).
alternatively, can build+install libraries independently (in bottom-up dependency order),

We call this last a 'vanilla build', since it follows standard practice for installing 3rd-party libraries. In a so-called vanilla build, we use cmake's find_package() support to acquire each dependency. In a vanilla build, When cmake builds a library, it obtains its dependencies from their final install location.

Contrast this with the submodule build: here, when cmake build a library, it obtains its dependencies from the build tree

Problem

submodule build works as expected; compile flags include required -Ipath/to/eigen/eigen3, so c++ code like this compiles
```
  #include <Eigen/Dense>
```
vanilla build fails: when compiling xo-pykalmanfilter, the header path for eigen is given as -Ipath/to/eigen instead of -Ipath/to/eigen/eigen3, so now need this to compile instead:
```
  #include <eigen3/Eigen/Dense>
```

Details

xo-kalmanfilter specifies eigen dependency in the approved manner:

  # xo-kalmanfilter/src/kalmanfilter/CMakeLists.txt
  set(SELF_LIB xo_kalmanfilter)
  ..
  xo_external_target_dependency(${SELF_LIB} Eigen3 Eigen3::Eigen)

which expands as if we had written:

  find_package(Eigen3 CONFIG REQUIRED)
  target_link_libraries(${SELF_LIB} PUBLIC Eigen3::Eigen)

This works as expected in submodule build. In submodule build, codebases xo-kalmanfilter and xo-pykalmanfilter (amongst others) are incorporated into a single source tree:

  # xo-sm2/CMakeLists.txt
  set(XO_SUBMODULE_BUILD True)
  ..
  add_subdirectory(repo/xo-kalmanfilter)
  add_subdirectory(repo/xo-pykalmanfilter)

xo-pykalmanfilter specifies xo-kalmanfilter dependency:

  # xo-pykalmanfilter/src/pykalmanfilter/CMakeLists.txt
  set(SELF_LIB pykalmanfilter)
  ..
  xo_pybind11_dependency(${SELF_LIB} xo_kalmanfilter)

which expands differently, depending on build type. In submodule build, as if we had written:

  target_include_directories(${SELF_LIB} PUBLIC $<BUILD_INTERFACE:${CMAKE_SOURCE_DIR}/repo/xo_kalmanfilter/include>)
  target_include_directories(${SELF_LIB} PUBLIC $<BUILD_INTERFACE:${CMAKE_BINARY_DIR}/repo/xo_kalmanfilter/include>)
  target_link_libraries(${SELF_LIB} PUBLIC xo_kalmanfilter)

In vanilla build, xo_pybind11_dependency() expands differently:

  find_package(xo_kalmanfilter CONFIG REQUIRED)
  ..
  target_link_libraries(${SELF_LIB} PUBLIC xo_kalmanfilter)

xo-kalmanfilter provides support for cmake find_package():

  # xo-kalmanfilter/cmake/xo_kalmanfilterConfig.cmake.in

  @PACKAGE_INIT@

  include(CMakeFindDependencyMacro)
  find_dependency(reactor)
  find_dependency(eigen3)

  include("${CMAKE_CURRENT_LIST_DIR}/@PROJECT_NAME@Targets.cmake")
  check_required_components("@PROJECT_NAME@")

and generated xo_kalmanfilterTargets.cmake file contains:

  # Create imported target xo_kalmanfilter
  add_library(xo_kalmanfilter SHARED IMPORTED)

  set_target_properties(xo_kalmanfilter PROPERTIES
    INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include;${_IMPORT_PREFIX}/include/xo/kalmanfilter"
    INTERFACE_LINK_LIBRARIES "reactor;Eigen3::Eigen"
  )

which.. doesn't look wrong :)

Evidence points to an inconsistency in Eigen-provided cmake support, if not in cmake proper.

Workaround

It's sufficient to restate the eigen dependency in xo-pykalmanfilter:

  # xo_pykalmanfilter/src/pykalmanfilter/CMakeLists.txt
  set(SELF_LIB pykalmanfilter)
  ...
  xo_external_target_dependency(${SELF_LIB} Eigen3 Eigen3::Eigen)

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