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

[Alexander Neundorf]

On Saturday 09 February 2013, Alexander Neundorf wrote:
> Hi,
> 
> a few weeks ago I added the feature that within one project, you can export
> depending targets into separate exports.
> When doing that, cmake now automatically adds a check whether the required
> targets (which were exported in a different export set) exist:
> 
> if(NOT TARGET foolib )
>    message(FATAL_ERROR "Required imported target foolib not found !")
> endif()
> 
> 
> This is nice.
> 
> But this is not done for other required targets, which were imported
> targets already in the exporting project (e.g. if libkcoreaddons has
> Qt5::Core in its LINK_INTERFACE).
> 
> IMO it would be nice if cmake automatically would create the same check as
> above for targets from the same project.
> I'm not sure whether this could cause build breakage... I think suddenly
> the order of the find_package() calls would become more important, the
> needed projects would have to be searched before...
> 
> Or is that already done now in some branch ?
> I lost a bit track of the other threads...

after having worked a bit more with depending projects which load each others 
exported targets, in the meantime I think this is a must, the exported target 
files must carry the information whether a name they use in the link interface 
is a library or a target, and cmake must fail if a target used there does not 
exist.

Example: I played around with libkconfig, which depends on libkcoreaddons.
Without the checking whether libkcoreaddons has been found already in the 
Config.cmake file, I successfully loaded kconfigConfig.cmake, which has 
"kcoreaddons" in its link interface.
So the cmake run succeeded.
It also compiled, because the headers were found.
But when linking I got "could not find "-lkcoreaddons".
This should not happen if the cmake finished successfully.

No normal user will figure out why things are going wrong.
I see two options:
1) either put a check for the required targets in the generated targets file, 
and make find_package() fail by setting foo_FOUND to FALSE. This should be 
relatively straightforward to implement, and needs only code in the generated 
targets file. This is good. On the downside, this will probably make the cmake 
run fail for (some ?) projects where it currently succeeds, although they do 
not take the dependency order into account when searching packages.

2) in the Config.cmake file, collect all referenced targets, and have a check 
at the end of the configure-step whether there are any referenced targets 
which have not been defined, and fail then. The good thing about this is that 
it will not break the cmake run of projects which are working right now, the 
downside is that the error message will probably be much less detailled than 
in the first case.

I'd prefer option 1), even though it may mean some short-term pain.
But maybe option 2) is already work in progress, since there was some talk 
about defined targets in the other thread ?

Comments ?

Alex