fix(cmake): correctly detect FindPython policy and better warning (#4806)

CMakeLists.txt

@@ -5,6 +5,11 @@
 # All rights reserved. Use of this source code is governed by a
 # All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 # BSD-style license that can be found in the LICENSE file.
 
 
+# Propagate this policy (FindPythonInterp removal) so it can be detected later
+if(NOT CMAKE_VERSION VERSION_LESS "3.27")
+  cmake_policy(GET CMP0148 _pybind11_cmp0148)
+endif()
+
 cmake_minimum_required(VERSION 3.5)
 cmake_minimum_required(VERSION 3.5)
 
 
 # The `cmake_minimum_required(VERSION 3.5...3.26)` syntax does not work with
 # The `cmake_minimum_required(VERSION 3.5...3.26)` syntax does not work with
@@ -16,6 +21,11 @@ else()
   cmake_policy(VERSION 3.26)
   cmake_policy(VERSION 3.26)
 endif()
 endif()
 
 
+if(_pybind11_cmp0148)
+  cmake_policy(SET CMP0148 ${_pybind11_cmp0148})
+  unset(_pybind11_cmp0148)
+endif()
+
 # Avoid infinite recursion if tests include this as a subdirectory
 # Avoid infinite recursion if tests include this as a subdirectory
 if(DEFINED PYBIND11_MASTER_PROJECT)
 if(DEFINED PYBIND11_MASTER_PROJECT)
   return()
   return()

tools/FindPythonLibsNew.cmake

@@ -95,6 +95,22 @@ if(NOT PythonLibsNew_FIND_VERSION)
   set(PythonLibsNew_FIND_VERSION "3.6")
   set(PythonLibsNew_FIND_VERSION "3.6")
 endif()
 endif()
 
 
+if(NOT CMAKE_VERSION VERSION_LESS "3.27")
+  cmake_policy(GET CMP0148 _pybind11_cmp0148)
+  if(NOT _pybind11_cmp0148)
+    message(
+      AUTHOR_WARNING
+        "Policy CMP0148 is not set: The FindPythonInterp and FindPythonLibs "
+        "modules are removed.  Run \"cmake --help-policy CMP0148\" for policy "
+        "details.  Use the cmake_policy command to set the policy and suppress "
+        "this warning, or preferably upgrade to using FindPython, either by "
+        "calling it explicitly before pybind11, or by setting "
+        "PYBIND11_FINDPYTHON ON before pybind11.")
+  endif()
+  cmake_policy(SET CMP0148 OLD)
+  unset(_pybind11_cmp0148)
+endif()
+
 find_package(PythonInterp ${PythonLibsNew_FIND_VERSION} ${_pythonlibs_required}
 find_package(PythonInterp ${PythonLibsNew_FIND_VERSION} ${_pythonlibs_required}
              ${_pythonlibs_quiet})
              ${_pythonlibs_quiet})