I have code like this:

if (CPM_SOURCE_CACHE)
    file(GLOB THIRD_PARTY_INCLUDES LIST_DIRECTORIES true "${CPM_SOURCE_CACHE}/**/include")
    include_directories("${CPM_SOURCE_CACHE}" ${THIRD_PARTY_INCLUDES})
endif()

Yes, this is a sort of a hack, but some third-party libraries can't be built without it.
But if my project used as third-party (via add_subdirectory()), I don't set CPM_SOURCE_CACHE and build fails.

How to get always correct real path where CPM saves third-party?

[
Probably: CPM_PACKAGE_${PACKAGE}_SOURCE_DIR?
]