How to export CMake targets with dependant include directories? - cmake

Question

I am trying to export CMake targets such that they also include the public include directories of dependant targets.
Let there be three separate but depending CMake projects:
foobar_result defines a data type and methods for this datatype in result.hpp
foobar_interface defines a header-only abstract class in foobar_interface.hpp which uses the datatype defined in foobar_result (#include <result.hpp>)
foobar_implA is an implementation (foobar_implA.cpp, foobar_implA.hpp) of the abstract class in foobar_interface (#include <foobar_interface.hpp>), and hence indirectly depends on the datatype definition (it #include <result.hpp> via #include <foobar_interface.hpp>)
What I want to achieve is that when foobar_implA is find_package(foobar_interface), it should automatically populate the include directories and libraries with the dependencies of foobar_interface so that foobar_implA does not have to discover foobar_result's include directories manually.
So far, I tried to specify the three CMake projects as follows.
1. foobar_result
foobar_result defines a standard library with headers:
cmake_minimum_required(VERSION 3.5)
project(foobar_result)
include(GNUInstallDirs)
add_library(result SHARED src/result.cpp)
target_include_directories(result PUBLIC
$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
$<INSTALL_INTERFACE:$<INSTALL_PREFIX>/${CMAKE_INSTALL_INCLUDEDIR}>
)
install(EXPORT ${PROJECT_NAME}
DESTINATION share/${PROJECT_NAME}/cmake
FILE ${PROJECT_NAME}Config.cmake
)
install(TARGETS result EXPORT ${PROJECT_NAME})
install(DIRECTORY ${CMAKE_SOURCE_DIR}/include/
DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
FILES_MATCHING PATTERN "*.hpp"
)
The exported foobar_resultConfig.cmake:
add_library(result SHARED IMPORTED)
set_target_properties(result PROPERTIES
INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
)
and foobar_resultConfig-noconfig.cmake:
set_target_properties(result PROPERTIES
IMPORTED_LOCATION_NOCONFIG "${_IMPORT_PREFIX}/lib/libresult.so"
IMPORTED_SONAME_NOCONFIG "libresult.so"
)
correctly define the includes and libraries.
2. foobar_interface
The interface uses the result library:
cmake_minimum_required(VERSION 3.5)
project(foobar_interface)
include(GNUInstallDirs)
find_package(foobar_result REQUIRED)
add_library(foobar_interface INTERFACE)
target_link_libraries(foobar_interface INTERFACE result)
target_include_directories(${PROJECT_NAME} INTERFACE
$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
$<INSTALL_INTERFACE:$<INSTALL_PREFIX>/${CMAKE_INSTALL_INCLUDEDIR}>
)
install(EXPORT ${PROJECT_NAME}
DESTINATION share/${PROJECT_NAME}/cmake
FILE ${PROJECT_NAME}Config.cmake
)
install(DIRECTORY ${CMAKE_SOURCE_DIR}/include/
DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
FILES_MATCHING PATTERN "*.hpp"
)
install(TARGETS ${PROJECT_NAME} EXPORT ${PROJECT_NAME})
Now, although the interface foobar_interface links the result library, the exported configuration:
add_library(foobar_interface INTERFACE IMPORTED)
set_target_properties(foobar_interface PROPERTIES
INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
INTERFACE_LINK_LIBRARIES "result"
)
only depends on the result library, but not its include directories. foobar_interface only exports its own INTERFACE_INCLUDE_DIRECTORIES, but not those of its dependency foobar_result.
3. foobar_implA
The implementation of the interface depends only on foobar_implA:
cmake_minimum_required(VERSION 3.5)
project(foobar_implA)
include(GNUInstallDirs)
find_package(foobar_interface REQUIRED)
add_library(foobar_implA SHARED src/foobar_implA.cpp)
target_link_libraries(foobar_implA PUBLIC foobar_interface)
target_include_directories(foobar_implA PUBLIC
$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
$<INSTALL_INTERFACE:$<INSTALL_PREFIX>/${CMAKE_INSTALL_INCLUDEDIR}>
)
install(EXPORT ${PROJECT_NAME}
DESTINATION share/${PROJECT_NAME}/cmake
FILE ${PROJECT_NAME}Config.cmake
)
install(DIRECTORY ${CMAKE_SOURCE_DIR}/include/
DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
FILES_MATCHING PATTERN "*.hpp"
)
install(TARGETS ${PROJECT_NAME} EXPORT ${PROJECT_NAME})
fails with:
In file included from [...]/src/foobar_implA/include/foobar_implA.hpp:3,
from [...]/src/foobar_implA/src/foobar_implA.cpp:1:
[...]/install/foobar_interface/include/foobar_interface.hpp:3:10: fatal error: result.hpp: No such file or directory
3 | #include <result.hpp>
| ^~~~~~~~~~~~
because find_package(foobar_interface) will not populate the include directories with the one defined in foobar_result.
I can only work around this issue by manually including foobar_result again:
find_package(foobar_result REQUIRED)
target_link_libraries(foobar_implA PUBLIC result)
but this is obviously not what I want, as this would not scale well when the dependencies in foobar_interface increase.
I am using colcon to separate the build and install spaces for all three projects. This way, all projects have to properly export and import their include and link directories. Installing all projects in the same prefix would also circumvent this problem, as foobar_interface include directories would also contain headers from foobar_result.