CMP0023

Plain and keyword target_link_libraries() signatures cannot be mixed.

CMake 2.8.12 introduced the target_link_libraries() signature using the PUBLIC, PRIVATE, and INTERFACE keywords to generalize the LINK_PUBLIC and LINK_PRIVATE keywords introduced in CMake 2.8.7. Use of signatures with any of these keywords sets the link interface of a target explicitly, even if empty. This produces confusing behavior when used in combination with the historical behavior of the plain target_link_libraries() signature. For example, consider the code:

target_link_libraries(mylib A)
target_link_libraries(mylib PRIVATE B)

After the first line the link interface has not been set explicitly so CMake would use the link implementation, A, as the link interface. However, the second line sets the link interface to empty. In order to avoid this subtle behavior CMake now prefers to disallow mixing the plain and keyword signatures of target_link_libraries() for a single target.

The OLD behavior for this policy is to allow keyword and plain target_link_libraries() signatures to be mixed. The NEW behavior for this policy is to not to allow mixing of the keyword and plain signatures.

This policy was introduced in CMake version 2.8.12. CMake version 3.19.0-rc3 warns when the policy is not set and uses OLD behavior. Use the cmake_policy() command to set it to OLD or NEW explicitly.