[7oct2023] cmake header-only dependencies

My original investigation (below) mistaken. It turns out I didn't understand that cmake error from target_link_libraries():

INTERFACE library can only be used with the INTERFACE keyword of target_link_libraries

applies to the depended-on library (3rd argument), not the depending library (1st argument).

Mistaken investigation below for posterity :)

• NOTE: also asked on stack overflow here
• cmake version 3.25.3

• Must introduce a header-only library like this:

  add_library(foo INTERFACE)
  

  (instead of add_library(foo SHARED) or add_library(foo STATIC))

• Must specify include directories for a header-only library like this:

  target_include_directories(
      foo INTERFACE
      $<INSTALL_INTERFACE:path/to/include>
      $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/path/to/include>)
  

  cmake enforces this explicitly; gives error

  target_include_directories may only set INTERFACE properties on INTERFACE targets
  

• Must specify dependency on a header-only library like this:

  target_link_libraries(bar INTERFACE foo))
  

  (instead of target_link_libraries(bar PUBLIC foo))

  cmake enforces this explicitly; gives error

  INTERFACE library can only be used with the INTERFACE keyword of target_link_libraries
  

• Expected behavior: this is sufficient for compilation of bar to tell compiler about include paths for foo:

  gcc -Ipath/to/foo bar.cpp
  

  This expectation is satisfied for regular non-INTERFACE libraries.

• compilation fails to supply include paths for depended-on foo when compiling depending-on bar, if bar is a regular library
• predicted cause:
  1. for an INTERFACE library, cmake uses property INTERFACE_INCLUDE_DIRECTORIES; it does not populate INCLUDE_DIRECTORIES.
  2. target_link_libraries when applied to a STATIC or SHARED target, picks up the INCLUDE_DIRECTORIES property for the depended-on target, while ignoring the INTERFACE_INCLUDE_DIRECTORIES property.

• when depending on a header-only library, explictly incorporate depended-on INTERFACE_INCLUDE_DIRECTORIES to INCLUDE_DIRECTORIES:

  macro(dependency_headeronly target dep)
      target_link_libraries(${target} INTERFACE ${dep})
  
      get_target_property(dependency_headeronly__tmp ${dep} INTERFACE_INCLUDE_DIRECTORIES)
      set_property(
          TARGET ${target}
          APPEND PROPERTY INCLUDE_DIRECTORIES ${dependency_headeronly__tmp})
  endmacro()