It should be possible for example to do this:

 "$<AND:${FOO},$<BOOL:${TGT}>,$<BOOL:$<TARGET_PROPERTY:${TGT},PROP>"

such that it works simliarly to the C code:

 if (foo && tgt && tgt->prop())
   {
   }

The example of generator expression code is a little bit contrived as
it could be written other ways with the same functionality. Nevertheless,
as these cases already work and are intentional, test for them.