[CMake] Can an option enforce a default, even if cache is present?

[Mario Emmenlauer]

On 27.11.18 17:13, Eric Noulard wrote:
> Le mar. 27 nov. 2018 à 14:50, Mario Emmenlauer <mario at emmenlauer.de 
> <mailto:mario at emmenlauer.de>> a écrit :
>     Dear all,
> 
>     I've just discovered that option() behaves differently than I anticipated.
>     After reading the docs and searching with google I'm still confused how to
>     achieve my desired behaviour.
> 
>     What I've just learned is that unspecified options take their cached value
>     and do *not* go back to their default value, if a cache exists. I assumed
>     that options take their default when not explicitly specified.
> 
> 
> The behavior of option() gained new behavior in CMake 3.13.
> May be it could help in your particular case: 
> https://cmake.org/cmake/help/v3.13/policy/CMP0077.html#policy:CMP0077
> 
> you'll depend on latest CMake though.
> 
> 
>     Now my problem: I could not find a way to get the behaviour I'd like. Is it
>     possible to enforce the default for an option when its not specified by the
>     user, even if a cache exists?
> 
> 
> You mean you did not manage to force the cache value?
> You can:
> set(VAR "default_value" CACHE FORCE)
> 
> or the problem is you cannot know whether if a value has been user-provided?

Sorry, I was not very precise! Your last point is the problem. I fail to know
when the option was user-provided and when it was cache-provided.

So here is what I'd like:

#> grep MYOPT CMakeLists.txt
option(MYOPT "Description" OFF)
#> cmake              # I want the option disabled, this works fine.
#> cmake -DMYOPT=ON   # I want the option enabled, this works fine.
#> cmake              # I want the option disabled (back to default),
                       # but I observe the option taken from cache, enabled.

Is there some way to achieve my desired behaviour? I tried without success
unset(MYOPT), unset(MYOPT CACHE), and set(MYOPT OFF) before option(), but
they all lead to different behaviour.

All the best,

     Mario