fix: allow subinterp support to be disabled

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

Author: henryiii

include/pybind11/detail/common.h

@@ -256,9 +256,13 @@
 
 // Slightly faster code paths are available when PYBIND11_HAS_SUBINTERPRETER_SUPPORT is *not*
 // defined, so avoid defining it for implementations that do not support subinterpreters. However,
-// defining it unnecessarily is not expected to break anything.
-#if PY_VERSION_HEX >= 0x030C0000 && !defined(PYPY_VERSION) && !defined(GRAALVM_PYTHON)
-#    define PYBIND11_HAS_SUBINTERPRETER_SUPPORT
+// defining it unnecessarily is not expected to break anything (other than old iOS targets).
+#ifndef PYBIND11_HAS_SUBINTERPRETER_SUPPORT
+#    if PY_VERSION_HEX >= 0x030C0000 && !defined(PYPY_VERSION) && !defined(GRAALVM_PYTHON)
+#        define PYBIND11_HAS_SUBINTERPRETER_SUPPORT 1
+#    else
+#        define PYBIND11_HAS_SUBINTERPRETER_SUPPORT 0
+#    endif
 #endif
 
 // 3.12 Compatibility

include/pybind11/detail/internals.h

@@ -324,7 +324,7 @@ inline std::atomic<int> &get_num_interpreters_seen() {
 
 template <typename InternalsType>
 inline std::unique_ptr<InternalsType> *&get_internals_pp() {
-#ifdef PYBIND11_HAS_SUBINTERPRETER_SUPPORT
+#if PYBIND11_HAS_SUBINTERPRETER_SUPPORT
     if (get_num_interpreters_seen() > 1) {
         // Internals is one per interpreter. When multiple interpreters are alive in different
         // threads we have to allow them to have different internals, so we need a thread_local.

include/pybind11/subinterpreter.h

@@ -15,7 +15,7 @@
 
 #include <stdexcept>
 
-#if !defined(PYBIND11_HAS_SUBINTERPRETER_SUPPORT)
+#if !PYBIND11_HAS_SUBINTERPRETER_SUPPORT
 #    error "This platform does not support subinterpreters, do not include this file."
 #endif
 

tests/pyproject.toml

@@ -15,6 +15,7 @@ build.verbose = true
 
 [tool.scikit-build.cmake.define]
 PYBIND11_FINDPYTHON = true
+CMAKE_COLOR_DIAGNOSTICS = "1"
 
 [tool.cibuildwheel]
 test-sources = ["tests", "pyproject.toml"]
@@ -24,3 +25,4 @@ environment.PIP_PREFER_BINARY = "1"
 ios.environment.IPHONEOS_DEPLOYMENT_TARGET = "17.0"
 ios.xbuild-tools = ["cmake", "ninja"]
 ios.environment.PIP_EXTRA_INDEX_URL = "https://pypi.anaconda.org/beeware/simple"
+ios.config-settings."cmake.define.CMAKE_CXX_FLAGS" = "-DPYBIND11_HAS_SUBINTERPRETER_SUPPORT=0"

tests/test_embed/test_subinterpreter.cpp

@@ -1,5 +1,5 @@
 #include <pybind11/embed.h>
-#ifdef PYBIND11_HAS_SUBINTERPRETER_SUPPORT
+#if PYBIND11_HAS_SUBINTERPRETER_SUPPORT
 #    include <pybind11/subinterpreter.h>
 
 // Silence MSVC C++17 deprecation warning from Catch regarding std::uncaught_exceptions (up to