# 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 3.19.3 warns when the policy is not set and uses `OLD` behavior. Use the `cmake_policy()` command to set it to `OLD` or `NEW` explicitly.

Note

The `OLD` behavior of a policy is `deprecated by definition` and may be removed in a future version of CMake.