Add an alternative method for cmake 3.15+ with fewer side-effects

Author: hjmallon

CMakeLists.txt

@@ -1,5 +1,7 @@
 # Python_FIND_REGISTRY and CMAKE_FIND_FRAMEWORK should be set to NEVER:
 # - https://cmake.org/cmake/help/v3.13/module/FindPython.html#hints
+# From 3.15 use Python_FIND_VIRTUALENV = ONLY as it has fewer side effects
+# - https://cmake.org/cmake/help/v3.15/module/FindPython.html#hints
 cmake_minimum_required(VERSION 3.13)
 
 project(hunter_venv)

cmake/template/Config.cmake.in

@@ -13,14 +13,21 @@ get_filename_component(
     ABSOLUTE
 )
 
-set(CMAKE_FIND_FRAMEWORK NEVER CACHE STRING "...")
-if(NOT "${CMAKE_FIND_FRAMEWORK}" STREQUAL "NEVER")
-  message(FATAL_ERROR "CMAKE_FIND_FRAMEWORK is not NEVER")
-endif()
+if(CMAKE_VERSION VERSION_LESS "3.15.0")
+  set(CMAKE_FIND_FRAMEWORK NEVER CACHE STRING "...")
+  if(NOT "${CMAKE_FIND_FRAMEWORK}" STREQUAL "NEVER")
+    message(FATAL_ERROR "CMAKE_FIND_FRAMEWORK is not NEVER")
+  endif()
 
-set(Python_FIND_REGISTRY NEVER CACHE STRING "...")
-if(NOT "${Python_FIND_REGISTRY}" STREQUAL "NEVER")
-  message(FATAL_ERROR "Python_FIND_REGISTRY is not NEVER")
+  set(Python_FIND_REGISTRY NEVER CACHE STRING "...")
+  if(NOT "${Python_FIND_REGISTRY}" STREQUAL "NEVER")
+    message(FATAL_ERROR "Python_FIND_REGISTRY is not NEVER")
+  endif()
+else()
+  set(Python_FIND_VIRTUALENV ONLY CACHE STRING "...")
+  if(NOT "${Python_FIND_VIRTUALENV}" STREQUAL "ONLY")
+    message(FATAL_ERROR "Python_FIND_VIRTUALENV is not ONLY")
+  endif()
 endif()
 
 if("@HUNTER_VENV_USE_2@")
@@ -32,9 +39,16 @@ if("@HUNTER_VENV_USE_2@")
     set(Python2_ROOT_DIR "${__hunter_venv_root}")
   endif()
 
-  set(Python2_FIND_REGISTRY NEVER CACHE STRING "...")
-  if(NOT "${Python2_FIND_REGISTRY}" STREQUAL "NEVER")
-    message(FATAL_ERROR "Python2_FIND_REGISTRY is not NEVER")
+  if(CMAKE_VERSION VERSION_LESS "3.15.0")
+    set(Python2_FIND_REGISTRY NEVER CACHE STRING "...")
+    if(NOT "${Python2_FIND_REGISTRY}" STREQUAL "NEVER")
+      message(FATAL_ERROR "Python2_FIND_REGISTRY is not NEVER")
+    endif()
+  else()
+    set(Python2_FIND_VIRTUALENV ONLY CACHE STRING "...")
+    if(NOT "${Python2_FIND_VIRTUALENV}" STREQUAL "ONLY")
+      message(FATAL_ERROR "Python2_FIND_VIRTUALENV is not ONLY")
+    endif()
   endif()
 
   find_package(Python "@HUNTER_VENV_PYTHON_VERSION@" EXACT QUIET REQUIRED)
@@ -50,9 +64,16 @@ if("@HUNTER_VENV_USE_3@")
     set(Python3_ROOT_DIR "${__hunter_venv_root}")
   endif()
 
-  set(Python3_FIND_REGISTRY NEVER CACHE STRING "...")
-  if(NOT "${Python3_FIND_REGISTRY}" STREQUAL "NEVER")
-    message(FATAL_ERROR "Python3_FIND_REGISTRY is not NEVER")
+  if(CMAKE_VERSION VERSION_LESS "3.15.0")
+    set(Python3_FIND_REGISTRY NEVER CACHE STRING "...")
+    if(NOT "${Python3_FIND_REGISTRY}" STREQUAL "NEVER")
+      message(FATAL_ERROR "Python3_FIND_REGISTRY is not NEVER")
+    endif()
+  else()
+    set(Python3_FIND_VIRTUALENV ONLY CACHE STRING "...")
+    if(NOT "${Python3_FIND_VIRTUALENV}" STREQUAL "ONLY")
+      message(FATAL_ERROR "Python3_FIND_VIRTUALENV is not ONLY")
+    endif()
   endif()
 
   find_package(Python "@HUNTER_VENV_PYTHON_VERSION@" EXACT QUIET REQUIRED)