Can you include more details:

• What does you CMakeLists.txt look like?
• What command do you use to execute CMake?
• What version CMake do you use?
• What command did you use to install?
• What does the line # checks I added into xtensor-blas cmake mean?

Hello, sorry for the lack of details.

Cmake version : 3.10 (from ubuntu 18.04 package manager).
My CMakeLists (I commented the rest of the CMakeLists.txt when I tried to debug, I still had the same issue):

cmake_minimum_required(VERSION 3.10)
project(cmake_test VERSION 0.1)

find_package(Eigen3  REQUIRED)
find_package(xtensor REQUIRED)
find_package(xtensor-blas REQUIRED)

cmake command : mkdir build && cd build && cmake ..

For xtensor-blas installation :

git clone https://github.com/xtensor-stack/xtensor-blas.git
cd xtensor-blas
mkdir build && cd build
cmake -DCMAKE_INSTALL_PREFIX=/home/narvorpi/work/narval/soft/install ..
make install

content of CMAKE_PREFIX_PATH : /home/narvorpi/work/narval/soft/install

# checks I added into xtensor-blas cmake : I added some lines to /home/narvorpi/work/narval/soft/install/lib/cmake/xtensor-blasConfig.cmake:45 to check if the target xtensor-blas was effectively created :

if(NOT TARGET xtensor-blas)
  include("${CMAKE_CURRENT_LIST_DIR}/xtensor-blasTargets.cmake")
  if(NOT TARGET xtensor-blas)
    message(STATUS "No xtensor-blas target")
  else()
    message(STATUS "xtensor-blas target exists") # this is displayed on configure
  endif()
  get_target_property(xtensor-blas_INCLUDE_DIRS xtensor-blas INTERFACE_INCLUDE_DIRECTORIES)
  find_dependency(BLAS REQUIRED)
  find_dependency(LAPACK REQUIRED)
  target_link_libraries(xtensor-blas INTERFACE ${BLAS_LIBRARIES} ${LAPACK_LIBRARIES}) #this is le line which fails
  if(${CMAKE_VERSION} VERSION_GREATER_EQUAL 3.8)
    target_compile_features(xtensor-blas INTERFACE cxx_std_14)
  endif()
endif()

I finally managed to make it work, but I had to edit the xtensor-blasConfig.cmake. However I am by no means a cmake wizard, so I really don't understand why this works but not the original code.
I xtensor-blasConfig.cmake, I replaced the following lines:

if(NOT TARGET xtensor-blas)
  include("${CMAKE_CURRENT_LIST_DIR}/xtensor-blasTargets.cmake")
  get_target_property(xtensor-blas_INCLUDE_DIRS xtensor-blas INTERFACE_INCLUDE_DIRECTORIES)
  find_dependency(BLAS REQUIRED)
  find_dependency(LAPACK REQUIRED)
  target_link_libraries(xtensor-blas INTERFACE ${BLAS_LIBRARIES} ${LAPACK_LIBRARIES}) #this is le line which fails
  if(${CMAKE_VERSION} VERSION_GREATER_EQUAL 3.8)
    target_compile_features(xtensor-blas INTERFACE cxx_std_14)
  endif()
endif()

by these ones:

if(NOT TARGET xtensor-blas)
  include("${CMAKE_CURRENT_LIST_DIR}/xtensor-blasTargets.cmake")
  get_target_property(xtensor-blas_INCLUDE_DIRS xtensor-blas INTERFACE_INCLUDE_DIRECTORIES)
  find_dependency(BLAS REQUIRED)
  find_dependency(LAPACK REQUIRED)
  # target_link_libraries(xtensor-blas INTERFACE ${BLAS_LIBRARIES} ${LAPACK_LIBRARIES})
  message(STATUS "${BLAS_LIBRARIES};${LAPACK_LIBRARIES}")
  set_target_properties(xtensor-blas PROPERTIES 
    INTERFACE_LINK_LIBRARIES "${BLAS_LIBRARIES};${LAPACK_LIBRARIES}"
  )
  if(${CMAKE_VERSION} VERSION_GREATER_EQUAL 3.8)
    # target_compile_features(xtensor-blas INTERFACE cxx_std_14)
    set_target_properties(xtensor-blas PROPERTIES
        INTERFACE_COMPILE_FEATURES cxx_std_14
    )
  endif()
endif()

If you can throw some light on this, I would really appreciate it.

I think it has to do with your CMake version not accepting a more modern CMake version. You did exactly the same as what there was before with simple a different 'syntax'. To increase robustness we could update the CMake config with your code, possibly with a switch based on the CMake version.

At the same time, it might be worth updating your CMake.

Ok. Thanks for your reply. I will look into it.

However, shouldn't the cmake_minimum_required(VERSION 3.1) in xtensor-blas contains a more recent version then ? (unless you update the xtensor-blasConfig of course).

Indeed we should keep the minimum required version consistent with the implementation. I think this requirement was left because of old versions of Ubuntu which should not be so much used now.

Ok. Thank you for your help !

@JohanMabille Do you know which CMake version supports what in terms of the target? I have seen this this problem before in several libraries, but I cannot find the right details in the CMake documentation.

Not really, I need to do some cmake archeology. We also need to check which version of cmake is installed on "recent" release of Ubuntu (or check if updating CMake on Ubuntu is still a nightmare).

I'm still so surprised that such old CMake's are shipped... But ok, we should deal with it.

Not really, I need to do some cmake archeology. We also need to check which version of cmake is installed on "recent" release of Ubuntu (or check if updating CMake on Ubuntu is still a nightmare).

fyi, I installed cmake from sources and it worked right away. (I installed on user side though. I did not try to update the system-installed cmake). Also, it solved the issue above.

Both target_link_libraries() and target_compile_features() are supposed to be in the lists file (not the config file). I created a PR (#193) to resolve this.