[cmake-developers] Adding automatic checks for required targets in target-export files ?

[Stephen Kelly]

Alexander Neundorf wrote:

> Another thing came to my mind.
> Let's say kguiaddons depends on kconfig, so it will do
> find_package(kconfig)
> I guess it should not search the packages kconfig depends on, but rely on
> kconfigConfig.cmake doing that.

Yes.

> 
> So far, so good.
> 
> Now, if we have some optional libraries used in let's say kguiaddons, this
> may give a dynamic list of targets we depend on in the Config.cmake file.

Yes.

> So I guess we have to propagate the used targets/package down to the top
> level CMakeLists.txt, which generates the Config.cmake file, and there
> configure some code into the Config.cmake file which can check (and
> find_package()) the actually used targets as necessary...

Yes. There probably doesn't have to be much propagation. Let's be concrete 
and pretent that kguiaddons has a hard dependency on qtgui, and an optional 
public dependency on qtsvg. The top-level CMakeLists file looks like this:

 find_package(Qt5Gui REQUIRED)
 find_package(Qt5Svg OPTIONAL)

 add_subdirectory(src)

 set(find_package_string "find_package(Qt5Gui REQUIRED)\n")
 if(Qt5Svg_FOUND)
    # Required by downstreams because we were (optionally) built with it
    set(find_package_string "find_package(Qt5Svg REQUIRED)\n")
 endif()

 # Really a wrapper macro is used
 configure_file(kguiaddonsConfig.cmake.in kguiaddonsConfig.cmake)

All dependencies generated in the config file should be required, because we 
can determine at upstream-cmake-time whether it really is required.

> 
> 
> Independent from that, as a safeguard, the exported target files should in
> some way make sure that they will really work.

Sure, why not :).

> They now have already checks that the files their own libraries references
> actually exist, they have a check that the imported targets from within
> the same project exist, but they do not yet have a check that the needed
> imported targets from other projects exist too.

Correct. 

> This should be there to make sure cmake report "NOT FOUND" at cmake time
> when a needed target is missing instead of failing with "could not find -
> lkcoreaddons" at link time.

Makes sense to me, and should be reasonably possible.

Thanks,

Steve.