# CMP0001

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()`.

`CMAKE_BACKWARDS_COMPATIBILITY` should no longer be used.

The behavior is to check `CMAKE_BACKWARDS_COMPATIBILITY` and present it to the user. The `NEW` behavior is to ignore `CMAKE_BACKWARDS_COMPATIBILITY` completely.

In CMake 2.4 and below the variable `CMAKE_BACKWARDS_COMPATIBILITY` was used to request compatibility with earlier versions of CMake. In CMake 2.6 and above all compatibility issues are handled by policies and the `cmake_policy()` command. However, CMake must still check `CMAKE_BACKWARDS_COMPATIBILITY` for projects written for CMake 2.4 and below.

This policy was introduced in CMake version 2.6.0. 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.