Add support for shared_ptr<const T> in py::init() with smart_holder (#5731)

* Add overload to enable `.def(py::init(&rtrn_shcp))`. Also uncomment `.def(py::init(&rtrn_uqcp))` and `.def(py::init(&rtrn_udcp))`, which happen to work already (not sure what change in the past made those work).

* Introduce `construct_from_shared_ptr()` helper for DRY-ness.

Author: rwgk

include/pybind11/detail/init.h

@@ -246,20 +246,38 @@ void construct(value_and_holder &v_h,
     v_h.type->init_instance(v_h.inst, &smhldr);
 }
 
-template <typename Class, detail::enable_if_t<is_smart_holder<Holder<Class>>::value, int> = 0>
-void construct(value_and_holder &v_h, std::shared_ptr<Cpp<Class>> &&shd_ptr, bool need_alias) {
-    PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(need_alias);
+template <typename PtrType, typename Class>
+void construct_from_shared_ptr(value_and_holder &v_h,
+                               std::shared_ptr<PtrType> &&shd_ptr,
+                               bool need_alias) {
+    static_assert(std::is_same<PtrType, Cpp<Class>>::value
+                      || std::is_same<PtrType, const Cpp<Class>>::value,
+                  "Expected (const) Cpp<Class> as shared_ptr pointee");
     auto *ptr = shd_ptr.get();
     no_nullptr(ptr);
     if (Class::has_alias && need_alias && !is_alias<Class>(ptr)) {
         throw type_error("pybind11::init(): construction failed: returned std::shared_ptr pointee "
                          "is not an alias instance");
     }
-    auto smhldr = smart_holder::from_shared_ptr(shd_ptr);
-    v_h.value_ptr() = ptr;
+    // Cast to non-const if needed, consistent with internal design
+    auto smhldr
+        = smart_holder::from_shared_ptr(std::const_pointer_cast<Cpp<Class>>(std::move(shd_ptr)));
+    v_h.value_ptr() = const_cast<Cpp<Class> *>(ptr);
     v_h.type->init_instance(v_h.inst, &smhldr);
 }
 
+template <typename Class, detail::enable_if_t<is_smart_holder<Holder<Class>>::value, int> = 0>
+void construct(value_and_holder &v_h, std::shared_ptr<Cpp<Class>> &&shd_ptr, bool need_alias) {
+    construct_from_shared_ptr<Cpp<Class>, Class>(v_h, std::move(shd_ptr), need_alias);
+}
+
+template <typename Class, detail::enable_if_t<is_smart_holder<Holder<Class>>::value, int> = 0>
+void construct(value_and_holder &v_h,
+               std::shared_ptr<const Cpp<Class>> &&shd_ptr,
+               bool need_alias) {
+    construct_from_shared_ptr<const Cpp<Class>, Class>(v_h, std::move(shd_ptr), need_alias);
+}
+
 template <typename Class, detail::enable_if_t<is_smart_holder<Holder<Class>>::value, int> = 0>
 void construct(value_and_holder &v_h,
                std::shared_ptr<Alias<Class>> &&shd_ptr,

tests/test_class_sh_factory_constructors.cpp

@@ -108,31 +108,23 @@ TEST_SUBMODULE(class_sh_factory_constructors, m) {
         .def("get_mtxt", get_mtxt<atyp_shmp>);
 
     py::classh<atyp_shcp>(m, "atyp_shcp")
-        // py::class_<atyp_shcp, std::shared_ptr<atyp_shcp>>(m, "atyp_shcp")
-        // class_: ... must return a compatible ...
-        // classh: ... cannot pass object of non-trivial type ...
-        // .def(py::init(&rtrn_shcp))
+        .def(py::init(&rtrn_shcp))
         .def("get_mtxt", get_mtxt<atyp_shcp>);
 
     py::classh<atyp_uqmp>(m, "atyp_uqmp")
         .def(py::init(&rtrn_uqmp))
         .def("get_mtxt", get_mtxt<atyp_uqmp>);
 
     py::classh<atyp_uqcp>(m, "atyp_uqcp")
-        // class_: ... cannot pass object of non-trivial type ...
-        // classh: ... cannot pass object of non-trivial type ...
-        // .def(py::init(&rtrn_uqcp))
+        .def(py::init(&rtrn_uqcp))
         .def("get_mtxt", get_mtxt<atyp_uqcp>);
 
     py::classh<atyp_udmp>(m, "atyp_udmp")
         .def(py::init(&rtrn_udmp))
         .def("get_mtxt", get_mtxt<atyp_udmp>);
 
     py::classh<atyp_udcp>(m, "atyp_udcp")
-        // py::class_<atyp_udcp, std::unique_ptr<atyp_udcp, sddc>>(m, "atyp_udcp")
-        // class_: ... must return a compatible ...
-        // classh: ... cannot pass object of non-trivial type ...
-        // .def(py::init(&rtrn_udcp))
+        .def(py::init(&rtrn_udcp))
         .def("get_mtxt", get_mtxt<atyp_udcp>);
 
     py::classh<with_alias, with_alias_alias>(m, "with_alias")

tests/test_class_sh_factory_constructors.py

@@ -13,11 +13,11 @@ def test_atyp_factories():
     # sert m.atyp_cptr().get_mtxt() == "Cptr"
     assert m.atyp_mptr().get_mtxt() == "Mptr"
     assert m.atyp_shmp().get_mtxt() == "Shmp"
-    # sert m.atyp_shcp().get_mtxt() == "Shcp"
+    assert m.atyp_shcp().get_mtxt() == "Shcp"
     assert m.atyp_uqmp().get_mtxt() == "Uqmp"
-    # sert m.atyp_uqcp().get_mtxt() == "Uqcp"
+    assert m.atyp_uqcp().get_mtxt() == "Uqcp"
     assert m.atyp_udmp().get_mtxt() == "Udmp"
-    # sert m.atyp_udcp().get_mtxt() == "Udcp"
+    assert m.atyp_udcp().get_mtxt() == "Udcp"
 
 
 @pytest.mark.parametrize(