`jsoncppConfig.cmake.in` uses `cmake_policy(VERSION 3.0...)` which CMake 4.0+ rejects

[xuhdev]

jsoncppConfig.cmake.in contains:

jsoncpp/jsoncppConfig.cmake.in

Line 2 in cdc8483

cmake_policy(VERSION 3.0...3.26)

CMake 4.0 has removed compatibility with policy versions below 3.5:

Compatibility with versions of CMake older than 3.5 has been removed.

So any project using find_package(jsoncpp) with CMake 4.0+ fails with:

CMake Error at .../jsoncppConfig.cmake:2 (cmake_policy):
  Compatibility with CMake < 3.5 has been removed from CMake.

This was reported in #1604 but was closed based on a suggestion to bump the max instead of the min. That was a misunderstanding -- the error is about the min (3.0), not the max. CMake 4.0 rejects any cmake_policy(VERSION) or cmake_minimum_required(VERSION) with a minimum below 3.5, regardless of the max.

[oold]

I don't think that is intended behavior on CMake's part. CMake should never fail if there is a supported policy version within the min-max range given to cmake_minimum_required(VERSION) or cmake_policy(VERSION).

Note that calls to those commands can still support older versions of CMake by using their VERSION arguments' <min>...<max> syntax. This requires only the <min> version of CMake, but when running a newer version, sets policies up to the <max> version.

[baylesj]

@oold unfortunately this is very much intended behavior in CMake, see https://cmake.org/cmake/help/latest/command/cmake_minimum_required.html.

Relevant section:

Changed in version 4.0: Compatibility with versions of CMake older than 3.5 is removed. Calls to cmake_minimum_required(VERSION) or cmake_policy(VERSION) that do not specify at least 3.5 as their policy version (optionally via ...<policy_max>) will produce an error in CMake 4.0 and above.

Bumped the master branch to 3.5 min and I think that's reasonable since 3.5 was released in 2016, giving a reasonable 10 year buffer of support.

If there is a critical need to support older versions, I am happy to revisit in the future.

[oold]

Relevant section:

Changed in version 4.0: Compatibility with versions of CMake older than 3.5 is removed. Calls to cmake_minimum_required(VERSION) or cmake_policy(VERSION) that do not specify at least 3.5 as their policy version (optionally via ...<policy_max>) will produce an error in CMake 4.0 and above.

Well, I mean... It says:

specify at least 3.5 as their policy version (optionally via ...<policy_max>)

That's why this claim is so surprising to me. The documentation says it should work if the supported version is within the given range.

[baylesj]

Agreed. I do think it's weird but am happy to move the min version to 3.5 and call it a day. I don't have that many cycles to spend on maintaining JsonCpp and want to make the most of them in terms of supporting the library's health.