Scope package options to avoid changing the local scope (#235)

* Fix #222

* Fix #222

* move policy change and local options to cpm_add_subdirectory

* change default policy as well

see https://gitlab.kitware.com/cmake/cmake/-/issues/20312

* add test

* update check to not use the NOT operator

(interestingly it works as expected locally)

* simplify test by not using options

* check options in tests

Co-authored-by: Lars Melchior <TheLartians@users.noreply.github.com>

test/unit/local_dependency/OptionsCMakeLists.txt.in

@@ -0,0 +1,33 @@
+cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
+
+project(CPMTest)
+
+# ---- Options ----
+
+option(ENABLE_TEST_COVERAGE "Enable test coverage" OFF)
+
+# ---- Dependencies ----
+
+include(@CPM_PATH@/CPM.cmake)
+
+CPMAddPackage(
+  NAME Dependency
+  SOURCE_DIR ${CMAKE_CURRENT_LIST_DIR}/dependency
+  OPTIONS "DEFINE_ALTERNATIVE_FUNCTION YES"
+  EXCLUDE_FROM_ALL YES
+)
+
+# ---- Dependencies ----
+
+alternative_dependency_function()
+
+# ---- Check parameters ----
+
+include(@CPM_PATH@/testing.cmake)
+
+message("DEFINE_ALTERNATIVE_FUNCTION: ${DEFINE_ALTERNATIVE_FUNCTION}")
+
+# this option is overridden by CPM.cmake
+ASSERT_NOT_DEFINED(DEFINE_ALTERNATIVE_FUNCTION)
+# this option is leaked by the dependency
+ASSERT_EQUAL(${LEAKED_OPTION} "OFF")

test/unit/local_dependency/dependency/CMakeLists.txt

@@ -1,3 +1,14 @@
-function(dependency_function)
-  message("called external method")
-endfunction()
+cmake_minimum_required(VERSION 3.0 FATAL_ERROR)
+
+option(DEFINE_ALTERNATIVE_FUNCTION "define the alternative function" OFF)
+option(LEAKED_OPTION "this option will be leaked to the outer scope" OFF)
+
+if(NOT DEFINE_ALTERNATIVE_FUNCTION)
+  function(dependency_function)
+    message("called external method")
+  endfunction()
+else()
+  function(alternative_dependency_function)
+    message("called alternative external method")
+  endfunction()
+endif()