Add find_package overrides that also work in CONFIG mode (#498)

Author: patstew

README.md

@@ -214,6 +214,13 @@ In the case that `find_package` requires additional arguments, the parameter `FI
 
 Note that this does not apply to dependencies that have been defined with a truthy `FORCE` parameter. These will be added as defined.
 
+### CPM_DONT_UPDATE_MODULE_PATH
+
+By default, CPM will override any `find_package` commands to use the CPM downloaded version.
+This is equivalent to the `OVERRIDE_FIND_PACKAGE` FetchContent option, which has no effect in CPM.
+To disable this behaviour set the `CPM_DONT_UPDATE_MODULE_PATH` option.
+This will not work for `find_package(CONFIG)` in CMake versions before 3.24.
+
 ### CPM_USE_NAMED_CACHE_DIRECTORIES
 
 If set, CPM use additional directory level in cache to improve readability of packages names in IDEs like CLion. It changes cache structure, so all dependencies are downloaded again. There is no problem to mix both structures in one cache directory but then there may be 2 copies of some dependencies.

cmake/CPM.cmake

@@ -162,7 +162,7 @@ set(CPM_SOURCE_CACHE
     CACHE PATH "Directory to download CPM dependencies"
 )
 
-if(NOT CPM_DONT_UPDATE_MODULE_PATH)
+if(${CMAKE_VERSION} VERSION_LESS "3.24.0" AND NOT CPM_DONT_UPDATE_MODULE_PATH)
   set(CPM_MODULE_PATH
       "${CMAKE_BINARY_DIR}/CPM_modules"
       CACHE INTERNAL ""
@@ -269,10 +269,24 @@ endfunction()
 # finding the system library
 function(cpm_create_module_file Name)
   if(NOT CPM_DONT_UPDATE_MODULE_PATH)
-    # erase any previous modules
-    file(WRITE ${CPM_MODULE_PATH}/Find${Name}.cmake
-         "include(\"${CPM_FILE}\")\n${ARGN}\nset(${Name}_FOUND TRUE)"
-    )
+    if(${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.24.0")
+      # Redirect find_package calls to the CPM package. This is what FetchContent does when you set OVERRIDE_FIND_PACKAGE.
+      # The CMAKE_FIND_PACKAGE_REDIRECTS_DIR works for find_package in CONFIG mode, unlike the Find${Name}.cmake fallback.
+      # https://cmake.org/cmake/help/latest/module/FetchContent.html#fetchcontent-find-package-integration-examples
+      string(TOLOWER ${Name} NameLower)
+      file(WRITE ${CMAKE_FIND_PACKAGE_REDIRECTS_DIR}/${NameLower}-config.cmake
+          "include(\"${CMAKE_CURRENT_LIST_DIR}/${NameLower}-extra.cmake\" OPTIONAL)\n"
+          "include(\"${CMAKE_CURRENT_LIST_DIR}/${Name}Extra.cmake\" OPTIONAL)\n"
+      )
+      file(WRITE ${CMAKE_FIND_PACKAGE_REDIRECTS_DIR}/${NameLower}-version.cmake
+          "set(PACKAGE_VERSION_COMPATIBLE TRUE)\n"
+          "set(PACKAGE_VERSION_EXACT TRUE)\n"
+      )
+    else()
+      file(WRITE ${CPM_MODULE_PATH}/Find${Name}.cmake
+          "include(\"${CPM_FILE}\")\n${ARGN}\nset(${Name}_FOUND TRUE)"
+      )
+    endif()
   endif()
 endfunction()