[cmake-developers] Issuing errors for faulty INTERFACE_INCLUDE_DIRECTORIES

[Brad King]

On 03/26/2013 02:53 PM, Robert Maynard wrote:
> My concern with this feature is the use case of a directory not existing at configure time, but which will exist at linking time. Primarily this would occur when an imported library is the result of an add_custom_command call which creates a new directory. While this might be an edge case, I was
> wondering if we should have a way to skip the verification on a target. 

It is not an edge case.  The add_dependencies command explicitly supports
adding "dependencies" of an imported target that then transitively become
dependencies of real targets that use it.  The idea is to have a hand-made
imported target that references files that will be produced at build time
by something like ExternalProject.  The hand-made imported target would do

 add_dependencies(imp_tgt ep_tgt)

to get build-time dependencies right.  However, that will also mean that
if one defines INTERFACE_INCLUDE_DIRECTORIES on imp_tgt the paths may not
exist during CMake configuration.

I suggest that to support this case we allow the hand-made imported target
to request that the include interface not be validated:

 set_property(TARGET imp_tgt PROPERTY
   INTERFACE_INCLUDE_DIRECTORIES_ALLOW_MISSING 1)

Ideas for a better name?

Alternatively one could solve this case with no additional CMake feature
by hiding the needed paths in $<1:...> generator expressions.  That is not
as explicit, but I would be okay with that as the recommended solution over
an awkwardly named property.

-Brad