Transfer reduced test here.

Reduced from a PyCLIF use case in the wild by @wangxf123456 (internal change cl/565476030).

tests/CMakeLists.txt

@@ -134,6 +134,7 @@ set(PYBIND11_TEST_FILES
     test_class_sh_trampoline_shared_from_this
     test_class_sh_trampoline_shared_ptr_cpp_arg
     test_class_sh_trampoline_unique_ptr
+    test_class_sh_unique_ptr_custom_deleter
     test_class_sh_unique_ptr_member
     test_class_sh_virtual_py_cpp_mix
     test_class_sh_void_ptr_capsule

tests/test_class_sh_unique_ptr_custom_deleter.cpp

@@ -0,0 +1,40 @@
+#include <pybind11/smart_holder.h>
+
+#include "pybind11_tests.h"
+
+#include <memory>
+
+namespace pybind11_tests {
+namespace class_sh_unique_ptr_custom_deleter {
+
+// Reduced from a PyCLIF use case in the wild by @wangxf123456.
+class Pet {
+public:
+    using Ptr = std::unique_ptr<Pet, std::function<void(Pet *)>>;
+
+    std::string name;
+
+    static Ptr New(const std::string &name) {
+        return Ptr(new Pet(name), std::default_delete<Pet>());
+    }
+
+private:
+    explicit Pet(const std::string &name) : name(name) {}
+};
+
+} // namespace class_sh_unique_ptr_custom_deleter
+} // namespace pybind11_tests
+
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(pybind11_tests::class_sh_unique_ptr_custom_deleter::Pet)
+
+namespace pybind11_tests {
+namespace class_sh_unique_ptr_custom_deleter {
+
+TEST_SUBMODULE(class_sh_unique_ptr_custom_deleter, m) {
+    py::classh<Pet>(m, "Pet").def_readwrite("name", &Pet::name);
+
+    m.def("create", &Pet::New);
+}
+
+} // namespace class_sh_unique_ptr_custom_deleter
+} // namespace pybind11_tests

tests/test_class_sh_unique_ptr_custom_deleter.py

@@ -0,0 +1,6 @@
+from pybind11_tests import class_sh_unique_ptr_custom_deleter as m
+
+
+def test_create():
+    pet = m.create("abc")
+    assert pet.name == "abc"