CMP0077

option() honors normal variables.

The option() command is typically used to create a cache entry to allow users to set the option. However, there are cases in which a normal (non-cached) variable of the same name as the option may be defined by the project prior to calling the option() command. For example, a project that embeds another project as a subdirectory may want to hard-code options of the subproject to build the way it needs.

For historical reasons in CMake 3.12 and below the option() command removes a normal (non-cached) variable of the same name when:

• a cache entry of the specified name does not exist at all, or
• a cache entry of the specified name exists but has not been given a type (e.g. via -D<name>=ON on the command line).

In both of these cases (typically on the first run in a new build tree), the option() command gives the cache entry type BOOL and removes any normal (non-cached) variable of the same name. In the remaining case that the cache entry of the specified name already exists and has a type (typically on later runs in a build tree), the option() command changes nothing and any normal variable of the same name remains set.

In CMake 3.13 and above the option() command prefers to do nothing when a normal variable of the given name already exists. It does not create or update a cache entry or remove the normal variable. The new behavior is consistent between the first and later runs in a build tree. This policy provides compatibility with projects that have not been updated to expect the new behavior.

When the option() command sees a normal variable of the given name:

• The OLD behavior for this policy is to proceed even when a normal variable of the same name exists. If the cache entry does not already exist and have a type then it is created and/or given a type and the normal variable is removed.
• The NEW behavior for this policy is to do nothing when a normal variable of the same name exists. The normal variable is not removed. The cache entry is not created or updated and is ignored if it exists.

This policy was introduced in CMake version 3.13. CMake version 3.17.5 warns when the policy is not set and uses OLD behavior. Use the cmake_policy() command to set it to OLD or NEW explicitly.