[CMake] Figuring out in a generator expression whether a name is a target

[Attila Krasznahorkay]

Dear All,

I'm trying to do something a bit more complicated, so please bear with me...

In our project we use a special kind of source code generator. Called a "dictionary generator". (https://root.cern.ch/how/how-use-reflex) To simplify the life of our users a bit, we provide a helper function for generating "dictionary libraries", which has a signature like:

atlas_add_dictionary( MyDictLibrary
                      MyDictLibrary/MyDictLibraryDict.h
                      MyDictLibrary/selection.xml
                      INCLUDE_DIRS ...
                      LINK_LIBRARIES ... )

Now, in order to make sure that the dictionary generation command succeeds, we need to pull in the public include paths of all the libraries that this library is supposed to be linked against, recursively. To do this, I do the following behind the scenes:

foreach( l ${ARG_LINK_LIBRARIES} )
   # Skip physical libraries:
   if( EXISTS ${l} )
      continue()
   endif()
   # Skip generator expressions:
   if( ${l} MATCHES "[$]<" )
      continue()
   endif()
   # Get the include directories of this library:
   list( APPEND includes "$<TARGET_PROPERTY:${l}:INTERFACE_INCLUDE_DIRECTORIES>" )
endforeach()

As long as the users only specify either library targets, or absolute library path names here, this works well. But I just ran into some issues where this function is called with a system library name like "rt". So the project generation bails, complaining that "rt" is not a target.

Would it be possible to figure out in a generator expression if a given name is a target or not? Clearly I can't make this decision in an

if( TARGET ${l} )

statement, as the target may only get defined after this function call.

For now I'll be able to work around this issue, but I wonder if it would be possible to provide a full-proof implementation that would not be killed by the users specifying a low-level system library like this in the future.

Cheers,
           Attila