[cmake-developers] A policy for Policies

[Brad King]

On 06/11/2015 02:20 PM, Stephen Kelly wrote:
> So, KDE4 as a platform breaks the cmake compatibility design for its 
> downstreams. At least ECM, used by KDE Frameworks 5, doesn't do that.

That is unfortunate :(

> I still think they should be unconditional warnings.

Yes, see below.

> I also think we should make them REQUIRED_* eventually, but I expect
> you now never will.

Either way we clearly need to do a better job at convincing projects to
port away from OLD behaviors.  One idea is that if a project does

 cmake_minimum_required(VERSION X.Y) # or cmake_policy(VERSION X.Y)

then we can make it an error to set to OLD any policy introduced in
version X.Y or earlier.  (For pre-3.4 policies this may have to be an
unconditional warning instead.)  Then N(=6?) releases later we make
it an unconditional warning regardless of cmake_minimum_required.

We could also make setting a policy to OLD a deprecation warning
for use with CMAKE_ERROR_DEPRECATED and CMAKE_WARN_DEPRECATED.
We should also have a way to optionally turn policy-no-set
warnings into errors to make them easy to find even in scripts
whose output no one ever reads.

Thanks,
-Brad