fix: follow rest of pybind11 closer with PYBIND11_HAS_SUBINTERPRETER_SUPPORT

include/pybind11/detail/common.h

@@ -257,11 +257,14 @@
 // 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 (other than old iOS targets).
+// This can be overridden by the user with -DPYBIND11_HAS_SUBINTERPRETER_SUPPORT=1 or 0
 #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
+#else
+#    if PYBIND11_HAS_SUBINTERPRETER_SUPPORT == 0
+#        undef PYBIND11_HAS_SUBINTERPRETER_SUPPORT
 #    endif
 #endif
 

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() {
-#if PYBIND11_HAS_SUBINTERPRETER_SUPPORT
+#ifdef 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 !PYBIND11_HAS_SUBINTERPRETER_SUPPORT
+#ifndef PYBIND11_HAS_SUBINTERPRETER_SUPPORT
 #    error "This platform does not support subinterpreters, do not include this file."
 #endif
 

tests/mod_per_interpreter_gil.cpp

@@ -12,5 +12,9 @@ PYBIND11_MODULE(mod_per_interpreter_gil,
                 py::multiple_interpreters::per_interpreter_gil()) {
     m.def("internals_at",
           []() { return reinterpret_cast<uintptr_t>(&py::detail::get_internals()); });
-    m.attr("PYBIND11_HAS_SUBINTERPRETER_SUPPORT") = PYBIND11_HAS_SUBINTERPRETER_SUPPORT;
+#ifdef PYBIND11_HAS_SUBINTERPRETER_SUPPORT
+    m.attr("defined_PYBIND11_HAS_SUBINTERPRETER_SUPPORT") = true;
+#else
+    m.attr("defined_PYBIND11_HAS_SUBINTERPRETER_SUPPORT") = false;
+#endif
 }

tests/mod_shared_interpreter_gil.cpp

@@ -9,5 +9,9 @@ namespace py = pybind11;
 PYBIND11_MODULE(mod_shared_interpreter_gil, m, py::multiple_interpreters::shared_gil()) {
     m.def("internals_at",
           []() { return reinterpret_cast<uintptr_t>(&py::detail::get_internals()); });
-    m.attr("PYBIND11_HAS_SUBINTERPRETER_SUPPORT") = PYBIND11_HAS_SUBINTERPRETER_SUPPORT;
+#ifdef PYBIND11_HAS_SUBINTERPRETER_SUPPORT
+    m.attr("defined_PYBIND11_HAS_SUBINTERPRETER_SUPPORT") = true;
+#else
+    m.attr("defined_PYBIND11_HAS_SUBINTERPRETER_SUPPORT") = false;
+#endif
 }

tests/test_embed/test_subinterpreter.cpp

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

tests/test_multiple_interpreters.py

@@ -27,7 +27,7 @@ def test_independent_subinterpreters():
 
     m = pytest.importorskip("mod_per_interpreter_gil")
 
-    if not m.PYBIND11_HAS_SUBINTERPRETER_SUPPORT:
+    if not m.defined_PYBIND11_HAS_SUBINTERPRETER_SUPPORT:
         pytest.skip("Does not have subinterpreter support compiled in")
 
     code = """
@@ -101,7 +101,7 @@ def test_dependent_subinterpreters():
 
     m = pytest.importorskip("mod_shared_interpreter_gil")
 
-    if not m.PYBIND11_HAS_SUBINTERPRETER_SUPPORT:
+    if not m.defined_PYBIND11_HAS_SUBINTERPRETER_SUPPORT:
         pytest.skip("Does not have subinterpreter support compiled in")
 
     code = """