[CMake] Avoiding error when using add_subdirectory twice on same path (CMake 2.6.4)

[Tyler Roscoe]

On Wed, Jun 17, 2009 at 09:07:19AM -0400, Brad King wrote:
>   # Top/CMakeLists.txt
>   add_subdirectory(A)
>   add_subdirectory(B)
> 
>   # A/CMakeLists.txt
>   set(foo 1)
>   add_subdirectory(../C ../C)
> 
>   # B/CMakeLists.txt
>   set(foo 0)
>   add_subdirectory(../C ../C)
> 
>   # C/CMakeLists.txt
>   if(foo)
>     # Which of these
>   else(foo)
>     # two options?
>   endif(foo)
> 
> Switching A and B on/off or switching their order can lead to
> surprising changes in behavior.

Agreed, but we obviously don't do anything like this.

> Even if it were a policy, any project that tries to do this would get
> the warning.  Yes, it could temporarily avoid the warning by setting
> the policy to OLD, but it should *still* be updated to the NEW
> behavior!  Again, policies are not used for choosing among desirable
> behaviors.

I don't understand this. Are we debating semantics (whether the
multiple-add_subdirectory() behavior is "desirable")? Are policies to be
used for choosing between undesirable, now-deprecated behavior and
desirable behavior?

It seems to me that 2.6.4 introduced some changed/improved some
behavior. CMake policies are designed to help insulate CMake users from
changes in behavior. Because I cannot insulate myself from this change,
I am stuck between not upgrading to the latest CMake and rewriting my
scripts. Isn't *this* the kind of thing policies are designed to
address?

Thanks,
tyler