# CMP0021

Fatal error on relative paths in `INCLUDE_DIRECTORIES` target property.

CMake 2.8.10.2 and lower allowed the `INCLUDE_DIRECTORIES` target property to contain relative paths. The base path for such relative entries is not well defined. CMake 2.8.12 issues a `FATAL_ERROR` if the `INCLUDE_DIRECTORIES` property contains a relative path.

The `OLD` behavior for this policy is not to warn about relative paths in the `INCLUDE_DIRECTORIES` target property. The `NEW` behavior for this policy is to issue a `FATAL_ERROR` if `INCLUDE_DIRECTORIES` contains a relative path.

This policy was introduced in CMake version 2.8.12. CMake version 3.26.0 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.