Bring in tests/test_classh_mock.cpp,py from smart_holder branch as-is (tests pass without any further changes).

tests/CMakeLists.txt

@@ -136,6 +136,7 @@ set(PYBIND11_TEST_FILES
     test_class_sh_unique_ptr_custom_deleter
     test_class_sh_unique_ptr_member
     test_class_sh_virtual_py_cpp_mix
+    test_classh_mock
     test_const_name
     test_constants_and_functions
     test_copy_move

tests/test_classh_mock.cpp

@@ -0,0 +1,71 @@
+#include "pybind11_tests.h"
+
+// The main purpose of this test is to ensure the suggested BOILERPLATE code block below is
+// correct.
+
+// Copy this block of code into your project.
+// Replace FOOEXT with the name of your project.
+// BOILERPLATE BEGIN
+#ifdef FOOEXT_USING_PYBIND11_SMART_HOLDER
+#    include <pybind11/smart_holder.h>
+#else
+#    include <pybind11/pybind11.h>
+PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
+template <typename type_, typename... options>
+using classh = class_<type_, options...>;
+PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)
+#    ifndef PYBIND11_SH_AVL
+#        define PYBIND11_SH_AVL(...) std::shared_ptr<__VA_ARGS__> // "Smart_Holder if AVaiLable"
+#    endif
+#    ifndef PYBIND11_SH_DEF
+#        define PYBIND11_SH_DEF(...) std::shared_ptr<__VA_ARGS__> // "Smart_Holder if DEFault"
+#    endif
+#    ifndef PYBIND11_SMART_HOLDER_TYPE_CASTERS
+#        define PYBIND11_SMART_HOLDER_TYPE_CASTERS(...)
+#    endif
+#    ifndef PYBIND11_TYPE_CASTER_BASE_HOLDER
+#        define PYBIND11_TYPE_CASTER_BASE_HOLDER(...)
+#    endif
+#endif
+// BOILERPLATE END
+
+namespace {
+struct FooUc {};
+struct FooUp {};
+struct FooSa {};
+struct FooSc {};
+struct FooSp {};
+} // namespace
+
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(FooUp)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(FooSp)
+
+PYBIND11_TYPE_CASTER_BASE_HOLDER(FooSa, std::shared_ptr<FooSa>)
+
+TEST_SUBMODULE(classh_mock, m) {
+    // Please see README_smart_holder.rst, in particular section
+    // Classic / Conservative / Progressive cross-module compatibility
+
+    // Uses std::unique_ptr<FooUc> as holder in Classic or Conservative mode, py::smart_holder in
+    // Progressive mode.
+    py::class_<FooUc>(m, "FooUc").def(py::init<>());
+
+    // Uses std::unique_ptr<FooUp> as holder in Classic mode, py::smart_holder in Conservative or
+    // Progressive mode.
+    py::classh<FooUp>(m, "FooUp").def(py::init<>());
+
+    // Always uses std::shared_ptr<FooSa> as holder.
+    py::class_<FooSa, std::shared_ptr<FooSa>>(m, "FooSa").def(py::init<>());
+
+    // Uses std::shared_ptr<FooSc> as holder in Classic or Conservative mode, py::smart_holder in
+    // Progressive mode.
+    py::class_<FooSc, PYBIND11_SH_DEF(FooSc)>(m, "FooSc").def(py::init<>());
+    // -------------- std::shared_ptr<FooSc> -- same length by design, to not disturb the
+    // indentation of existing code.
+
+    // Uses std::shared_ptr<FooSp> as holder in Classic mode, py::smart_holder in Conservative or
+    // Progressive mode.
+    py::class_<FooSp, PYBIND11_SH_AVL(FooSp)>(m, "FooSp").def(py::init<>());
+    // -------------- std::shared_ptr<FooSp> -- same length by design, to not disturb the
+    // indentation of existing code.
+}

tests/test_classh_mock.py

@@ -0,0 +1,13 @@
+from __future__ import annotations
+
+from pybind11_tests import classh_mock as m
+
+
+def test_foobar():
+    # Not really testing anything in particular. The main purpose of this test is to ensure the
+    # suggested BOILERPLATE code block in test_classh_mock.cpp is correct.
+    assert m.FooUc()
+    assert m.FooUp()
+    assert m.FooSa()
+    assert m.FooSc()
+    assert m.FooSp()