What is the difference between “option” and “set CACHE BOOL” for a CMake variable?

Question

Is there any difference between the following two?

set(FOO true CACHE BOOL \"description\")

option(FOO \"description\" ON)

Documentation:

Answer 1

Stumbled on this question, and thought I added an update.

As explained here, the option command does not create a CACHE variable if a normal variable with the same name exists. This behavior was introduced in version 3.13 (where it is the default). On the other hand, setting a CACHE variable named FOO when FOO exists as a normal variable, will yield two copies of FOO, a normal variable and a CACHE one.

Therefore, whether to use option(FOO "" ON) or set(FOO ON CACHE BOOL "") may also depend on what you want your configuration system to do when a variable with the same name has already been defined upstream.

Note: the two still behave the same way if FOO has been defined upstream in the cache. The difference is only if the upstream is a normal variable.