System:

Mac OSX 15.0
apple clang 16.0.0
cmake 3.30.1

In recent versions of CMake the procedures for finding python have changed.

Suppose I have the following directory setup

extern/vcpkg/  # cloned from github and bootstrapped
extern/pybind11/  # cloned from github
CMakeLists.txt
main.cpp

With the following contents:

# CMakeLists.txt

# cmake_minimum_required(VERSION 3.26.0 FATAL_ERROR)
cmake_minimum_required(VERSION 3.27.0 FATAL_ERROR)
set(CMAKE_CXX_STANDARD 20)
set(CMAKE_EXPORT_COMPILE_COMMANDS 1)

# set (CMAKE_TOOLCHAIN_FILE ${CMAKE_SOURCE_DIR}/extern/vcpkg/scripts/buildsystems/vcpkg.cmake)

set(PROJECT mytest)
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_SOURCE_DIR})
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_SOURCE_DIR})
project(${PROJECT})
add_subdirectory(extern/pybind11)

pybind11_add_module(${PROJECT} main.cpp)

// main.cpp

#include <pybind11/pybind11.h>
#include <iostream>

void say_hello(){ std::cout << "Hello world!" << std::endl; }

PYBIND11_MODULE(mytest, m) { m.def("say_hello", &say_hello); }

Then I build with:

rm -rf build || true && cmake -H. -Bbuild && cmake --build build --target all

Under this configuration, the module builds and runs just fine

If I uncomment the CAMKE_TOOLCHAIN_FILE line, then the build fails with the following message:

CMake Error at extern/vcpkg/scripts/buildsystems/vcpkg.cmake:859 (_find_package):
  By not providing "FindPythonInterp.cmake" in CMAKE_MODULE_PATH this project
  has asked CMake to find a package configuration file provided by
  "PythonInterp", but CMake did not find one.

  Could not find a package configuration file provided by "PythonInterp"
  (requested version 3.8) with any of the following names:

    PythonInterpConfig.cmake
    pythoninterp-config.cmake

  Add the installation prefix of "PythonInterp" to CMAKE_PREFIX_PATH or set
  "PythonInterp_DIR" to a directory containing one of the above files.  If
  "PythonInterp" provides a separate development package or SDK, be sure it
  has been installed.
Call Stack (most recent call first):
  extern/pybind11/tools/FindPythonLibsNew.cmake:114 (find_package)
  extern/vcpkg/scripts/buildsystems/vcpkg.cmake:859 (_find_package)
  extern/pybind11/tools/pybind11Tools.cmake:44 (find_package)
  extern/pybind11/tools/pybind11Common.cmake:200 (include)
  extern/pybind11/CMakeLists.txt:229 (include)

If I then revert the minimum required version of cmake, then the policy settings change and the build succeeds again but with the following warning from pybind:

CMake Warning (dev) at extern/pybind11/tools/FindPythonLibsNew.cmake:101 (message):
  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.
Call Stack (most recent call first):
  extern/vcpkg/scripts/buildsystems/vcpkg.cmake:859 (_find_package)
  extern/pybind11/tools/pybind11Tools.cmake:44 (find_package)
  extern/pybind11/tools/pybind11Common.cmake:200 (include)
  extern/pybind11/CMakeLists.txt:229 (include)

I don’t really know what vcpkg is going to affect the settings here, so if someone knows, that would be useful to understand. Furthermore, I’m trying to understand if there is anything I can do to fix this problem so that I can use up to date policy settings