CMake 3 to 4: Compatibility with CMake < 3.5 has been removed

CMake keeps old behaviour through policies, which cmake_minimum_required(VERSION x) sets to their state at version x. CMake 4.0 removed the old behaviour of all policies introduced before 3.5, so any project, or vendored dependency, that declares a minimum below 3.5 stops at configure time. Many older libraries still have cmake_minimum_required(VERSION 2.8) or 3.1.

What CMake 4 does with old minimums

CMake 4.4 from the cmake Python wheel, three minimum versions:

cmake() { uv run -q --no-project --python 3.13 --with cmake==4.4.4 --with ninja cmake "$@"; }
cmake --version | head -1
for v in 3.0 3.5 3.10; do
  printf 'cmake_minimum_required(VERSION %s)\nproject(demo LANGUAGES NONE)\n' "$v" > CMakeLists.txt
  rm -rf build
  echo "=== VERSION $v"
  cmake -G Ninja -S . -B build 2>&1 | grep -E 'Error|Warning|Compatibility|Configuring|Generating done' ; echo "exit status: ${PIPESTATUS[0]}"
done

cmake version 4.4.4
=== VERSION 3.0
CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.
-- Configuring incomplete, errors occurred!
exit status: 1
=== VERSION 3.5
CMake Warning (deprecated) at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of
-- Configuring done (0.1s)
-- Generating done (0.0s)
exit status: 0
=== VERSION 3.10
-- Configuring done (0.1s)
-- Generating done (0.0s)
exit status: 0

Fixes

# your own project: declare what you actually test with
cmake_minimum_required(VERSION 3.16...4.4)

The min...max form says the project needs at least min and was updated for the policies up to max, so new CMake versions do not warn.

For a third-party project you cannot edit, configure anyway with a policy minimum:

cmake -S . -B build -DCMAKE_POLICY_VERSION_MINIMUM=3.5

Or set the environment variable CMAKE_POLICY_VERSION_MINIMUM=3.5, which also reaches dependencies built through FetchContent or ExternalProject. Better still, update the dependency to a release that declares a newer minimum.