[CMake] Populate INCLUDE_DIRECTORIES of object libraries from INTERFACE_INCLUDE_DIRECTORIES of other libraries

[Florent Castelli]

Which version of CMake and Ninja are you using?
It seems like you are trying to reimplement what has been added in CMake 
3.9. Quote from the changelog:

The Ninja generator has loosened the dependencies of object compilation. 
Object compilation now depends only on custom targets and custom 
commands associated with libraries on which the object’s target depends 
and no longer depends on the libraries themselves. Source files in 
dependent targets may now compile without waiting for their targets’ 
dependencies to link.

Have you tried it? It shouldn't need any change in your project, but I'm 
not sure how it will behave if you already started to migrate to OBJECT 
libraries.

Regards,
Florent

On 14/11/2017 17:48, Bernhard Burgermeister wrote:
> Hello,
>
> I'm trying to improve parallelization of a massive parallel
> Ninja/icecream build by using object libraries. We are already using
> interface include directories extensively.
>
> While moving the compilation to separate object libraries I need to
> define the include directories for these object targets. Previously
> this simply worked by target_link_library(x PRIVATE a) so that x uses
> the public and interface include directories of a. With object
> libraries this is not possible, for example
>
> ======================================
> CMAKE_MINIMUM_REQUIRED(VERSION 3.7)
> PROJECT(ObjectLibLink C)
>
> ADD_LIBRARY(a INTERFACE)
> TARGET_INCLUDE_DIRECTORIES(a INTERFACE include)
>
> ADD_LIBRARY(b OBJECT b.c)
> TARGET_LINK_LIBRARIES(b PRIVATE a)
> ======================================
>
> gives the error message
>    Object library target "b" may not link to anything
> with CMake 3.7, 3.9 and 3.10rc3. The documentation of the later
> version says "OBJECT libraries may not be used in the right hand side
> of target_link_libraries()" but does not restrict the left hand side.
> I would expect it to build b with include-directory "include" from a.
>
> Currently I'm using a workaround
>     FOREACH(Lib ${ARG_LINK})
>        IF(TARGET ${Lib})
>           TARGET_INCLUDE_DIRECTORIES(MyObjectLib PRIVATE
> $<TARGET_PROPERTY:${Lib},INTERFACE_INCLUDE_DIRECTORIES>)
>        ENDIF()
>     ENDFOREACH()
> but this only works for targets previously defined or only target
> libraries may be used. Otherwise the generator fails for file names or
> system library names because there is no generator expression to check
> for existing targets (see
> https://gitlab.kitware.com/cmake/cmake/issues/17123). I want to avoid
> explicitly import all these libraries.
>
>
> Is there an other reliable way to populate include_directories that I
> have not yet found? Of course compile_definitions and compile_options
> should be added from the other libraries too.
>
>
> Regards,
> Bernhard.