Support loading unique_ptr<Derived> as unique_ptr<Base>. (#4031)

* Support loading unique_ptr<derived> as unique_ptr<base>.

* Fix incorrect test

* pre commit fix

* Fix clang tidy

* Resolve comments

* Resolve comments

Author: wangxf123456

include/pybind11/detail/smart_holder_poc.h

@@ -181,7 +181,9 @@ struct smart_holder {
                                             + ").");
             }
             ensure_vptr_is_using_builtin_delete(context);
-        } else if (!(*rtti_requested == *rtti_uqp_del)) {
+        } else if (!(*rtti_requested == *rtti_uqp_del)
+                   && !(vptr_is_using_builtin_delete
+                        && is_std_default_delete<T>(*rtti_requested))) {
             throw std::invalid_argument(std::string("Incompatible unique_ptr deleter (") + context
                                         + ").");
         }

tests/pure_cpp/smart_holder_poc_test.cpp

@@ -37,6 +37,15 @@ struct indestructible_int {
     ~indestructible_int() = default;
 };
 
+struct base {
+    virtual int get() { return 10; }
+    virtual ~base() = default;
+};
+
+struct derived : public base {
+    int get() override { return 100; }
+};
+
 } // namespace helpers
 
 TEST_CASE("from_raw_ptr_unowned+as_raw_ptr_unowned", "[S]") {
@@ -227,6 +236,25 @@ TEST_CASE("from_unique_ptr+as_shared_ptr", "[S]") {
     REQUIRE(*new_owner == 19);
 }
 
+TEST_CASE("from_unique_ptr_derived+as_unique_ptr_base", "[S]") {
+    std::unique_ptr<helpers::derived> orig_owner(new helpers::derived());
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    std::unique_ptr<helpers::base> new_owner = hld.as_unique_ptr<helpers::base>();
+    REQUIRE(!hld.has_pointee());
+    REQUIRE(new_owner->get() == 100);
+}
+
+TEST_CASE("from_unique_ptr_derived+as_unique_ptr_base2", "[E]") {
+    std::unique_ptr<helpers::derived, helpers::functor_other_delete<helpers::derived>> orig_owner(
+        new helpers::derived());
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    REQUIRE_THROWS_WITH(
+        (hld.as_unique_ptr<helpers::base, helpers::functor_builtin_delete<helpers::base>>()),
+        "Incompatible unique_ptr deleter (as_unique_ptr).");
+}
+
 TEST_CASE("from_unique_ptr_with_deleter+as_lvalue_ref", "[S]") {
     std::unique_ptr<int, helpers::functor_builtin_delete<int>> orig_owner(new int(19));
     auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));