conan-provider.cmake: introduce a new policy scope, set min cmake version (#645)

cmake has the notion of "policy scopes" to limit the scope of the policies.
this patch introduces a module-level policy scope and sets the minimum
required cmake version to 3.24, so the cmake features depend on policy settings
such as `if(... IN_LIST ...)` behaves as expected even if the consuming project
does not declare a cmake minimum version requirement. also, this change now
enforces the version requirement so the code would fail immediately if the
cmake version is less than the minimum.

Signed-off-by: Mustafa Kemal Gilor <hello@mkg.dev>

Author: xmkg

conan_provider.cmake

@@ -22,6 +22,18 @@
 
 set(CONAN_MINIMUM_VERSION 2.0.5)
 
+# Create a new policy scope and set the minimum required cmake version so the
+# features behind a policy setting like if(... IN_LIST ...) behaves as expected
+# even if the parent project does not specify a minimum cmake version or a minimum
+# version less than this module requires (e.g. 3.0) before the first project() call.
+# (see: https://cmake.org/cmake/help/latest/variable/CMAKE_PROJECT_TOP_LEVEL_INCLUDES.html)
+#
+# The policy-affecting calls like cmake_policy(SET...) or `cmake_minimum_required` only
+# affects the current policy scope, i.e. between the PUSH and POP in this case.
+#
+# https://cmake.org/cmake/help/book/mastering-cmake/chapter/Policies.html#the-policy-stack
+cmake_policy(PUSH)
+cmake_minimum_required(VERSION 3.24)
 
 function(detect_os OS OS_API_LEVEL OS_SDK OS_SUBSYSTEM OS_VERSION)
     # it could be cross compilation
@@ -647,3 +659,5 @@ if(NOT _cmake_program)
     get_filename_component(PATH_TO_CMAKE_BIN "${CMAKE_COMMAND}" DIRECTORY)
     set(PATH_TO_CMAKE_BIN "${PATH_TO_CMAKE_BIN}" CACHE INTERNAL "Path where the CMake executable is")
 endif()
+
+cmake_policy(POP)

tests/resources/find_module/policy_scope/CMakeLists.txt

@@ -0,0 +1,21 @@
+cmake_minimum_required(VERSION 3.0)
+project(MyApp CXX)
+
+set(CMAKE_CXX_STANDARD 17)
+find_package(hello REQUIRED)
+# Request that Findbye.cmake (generated by Conan)
+# is used instead of bye-config.cmake
+find_package(bye MODULE REQUIRED)
+
+# The `find_package` will include the conan_provider.cmake file
+# which has policy-affecting code. We expect no changes to the current
+# policy scope.
+
+add_executable(app main.cpp)
+target_link_libraries(app hello::hello bye::bye)
+
+set(test_list a b c d e f)
+if("a" IN_LIST test_list)
+    # see https://cmake.org/cmake/help/latest/policy/CMP0057.html
+    message(ERROR "this should not happen as IN_LIST is a CMake 3.3 feature.")
+endif()

tests/test_smoke.py

@@ -277,6 +277,23 @@ def test_cmake_builtin_module(self, capfd, basic_cmake_project):
         out, _ = capfd.readouterr()
         assert "Found Threads: TRUE" in out
 
+    def test_policy_scope(self, capfd, basic_cmake_project):
+        """
+        Ensure that the policy settings of the user project is not affected by the
+        "module's policy-affecting calls.
+        """
+        source_dir, binary_dir = basic_cmake_project
+        shutil.copytree(resources_dir / 'find_module' / 'policy_scope', source_dir, dirs_exist_ok=True)
+
+        run(f"cmake -S {source_dir} -B {binary_dir} -DCMAKE_PROJECT_TOP_LEVEL_INCLUDES={conan_provider} -DCMAKE_BUILD_TYPE=Release", check=False)
+        out, err = capfd.readouterr()
+        assert not re.search(rf'CMake Error at {re.escape(str(conan_provider))}.*\(if\):\n  if given arguments', err)
+        assert "Conan: Target declared 'hello::hello'" in out
+        assert "Conan: Target declared 'bye::bye'" in out
+        assert "\"a\" \"IN_LIST\" \"test_list\"" in err
+        assert "this should not happen as IN_LIST is a CMake 3.3 feature." not in err
+
+
 
 class TestCMakeModulePath: