# CMP0021

Warning

The `OLD` behavior of this policy was removed in CMake version 4.0. This policy must be set to `NEW` by a call to `cmake_minimum_required()` or `cmake_policy()`.

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. Prior to removal in CMake version 4.0, it could be set by `cmake_policy()` or `cmake_minimum_required()`. If it was not set, CMake warned, and used `OLD` behavior.