Change macro name to PYBIND11_RUN_TESTING_WITH_SMART_HOLDER_AS_DEFAULT_BUT_NEVER_USE_IN_PRODUCTION_PLEASE

Author: rwgk

.github/workflows/ci.yml

@@ -88,20 +88,20 @@ jobs:
           # Extra ubuntu latest job
           - runs-on: ubuntu-latest
             python: '3.11'
-          # Exercise PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
+          # Run tests with py::smart_holder as the default holder
           # with recent (or ideally latest) released Python version.
           - runs-on: ubuntu-latest
             python: '3.12'
             args: >
-              -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+              -DCMAKE_CXX_FLAGS="-DPYBIND11_RUN_TESTING_WITH_SMART_HOLDER_AS_DEFAULT_BUT_NEVER_USE_IN_PRODUCTION_PLEASE"
           - runs-on: macos-13
             python: '3.12'
             args: >
-              -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+              -DCMAKE_CXX_FLAGS="-DPYBIND11_RUN_TESTING_WITH_SMART_HOLDER_AS_DEFAULT_BUT_NEVER_USE_IN_PRODUCTION_PLEASE"
           - runs-on: windows-2022
             python: '3.12'
             args: >
-              -DCMAKE_CXX_FLAGS="/DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT /GR /EHsc"
+              -DCMAKE_CXX_FLAGS="/DPYBIND11_RUN_TESTING_WITH_SMART_HOLDER_AS_DEFAULT_BUT_NEVER_USE_IN_PRODUCTION_PLEASE /GR /EHsc"
         exclude:
           # The setup-python action currently doesn't have graalpy for windows
           - python: 'graalpy-24.1'

include/pybind11/pybind11.h

@@ -1839,14 +1839,13 @@ struct property_cpp_function<
         detail::both_t_and_d_use_type_caster_base<T, typename D::element_type>>::value>>
     : detail::property_cpp_function_sh_unique_ptr_member<T, D> {};
 
-#if defined(PYBIND11_USE_SMART_HOLDER_AS_DEFAULT)
-// NOTE: THIS IS MEANT FOR STRESS-TESTING ONLY!
-//       As of PR #5257, for production use, there is no longer a strong reason to make
-//       smart_holder the default holder:
-//           Simply use `py::classh` (see below) instead of `py::class_` as needed.
+#ifdef PYBIND11_RUN_TESTING_WITH_SMART_HOLDER_AS_DEFAULT_BUT_NEVER_USE_IN_PRODUCTION_PLEASE
+// NOTE: THIS IS MEANT FOR STRESS-TESTING OR TRIAGING ONLY!
 //       Running the pybind11 unit tests with smart_holder as the default holder is to ensure
 //       that `py::smart_holder` / `py::classh` is backward-compatible with all pre-existing
 //       functionality.
+//       Be careful not to link translation units compiled with different default holders, because
+//       this will cause ODR violations (https://en.wikipedia.org/wiki/One_Definition_Rule).
 template <typename>
 using default_holder_type = smart_holder;
 #else

tests/test_class.cpp

@@ -632,7 +632,7 @@ CHECK_NOALIAS(8);
 CHECK_HOLDER(1, unique);
 CHECK_HOLDER(2, unique);
 CHECK_HOLDER(3, unique);
-#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
+#ifndef PYBIND11_RUN_TESTING_WITH_SMART_HOLDER_AS_DEFAULT_BUT_NEVER_USE_IN_PRODUCTION_PLEASE
 CHECK_HOLDER(4, unique);
 CHECK_HOLDER(5, unique);
 #endif