[cmake-developers] target_link_libraries not callable from other directory scopes

[Patrick Stotko]

Am 27.04.2018 um 15:13 schrieb Brad King:
> On 04/26/2018 07:18 PM, Craig Scott wrote:
>> Perhaps it was an oversight that newer target_... commands don't have the same
>> restriction as target_link_libraries(), but it is a very useful oversight!
>> As the linked blog article explains, it allows much better modularity of the
>> project.
> Yes, and usage requirements make non-local effects commonplace anyway.
> Those didn't exist when the `target_link_libraries` restriction was
> first put in place.  Back then *everything* that affected a target's
> build was in its own `CMakeLists.txt` file.
>
>> Being able to use add_subdirectory() instead of include() allows the subdirectory
>> to be more isolated
> The original restriction was to prevent parent and sibling directories from
> affecting a target's build.  We didn't think much about subdirectories as
> a way of incrementally accumulating build information for a target.
>
> I think it would be fine to lift the restriction if there is no technical
> hurdle.
>
> -Brad

As far as I can see, the code that prevents linking from other 
directories starts here: 
https://gitlab.kitware.com/cmake/cmake/blob/master/Source/cmTargetLinkLibrariesCommand.cxx#L369
The check for imported targets should be definitly kept since users 
should still not be able to modify third party targets. Maybe 
FindLocalNonAliasTarget() can be replaced with something like 
FindNonAliasTarget() or FindGlobalNonAliasTarget() to catch aliases. I 
am not sure whether such a thing already exists or must be implemented.

We can also continue the discussion at GitLab to avoid bothering the 
others with such technical details if you like.

Best regards,
Patrick Stotko