Add unittest checking drake#11424

Remove id check for newer result

tests/test_class.cpp

@@ -467,6 +467,24 @@ TEST_SUBMODULE(class_, m) {
         py::class_<OtherDuplicateNested>(gt, "OtherDuplicateNested");
         py::class_<OtherDuplicateNested>(gt, "YetAnotherDuplicateNested");
     });
+
+    // Test drake#11424.
+    class ExampleVirt2 {
+        public:
+            virtual ~ExampleVirt2() {}
+            virtual std::string get_name() const { return "ExampleVirt2"; }
+        };
+    class PyExampleVirt2 : public ExampleVirt2 {
+    public:
+        std::string get_name() const override {
+            PYBIND11_OVERLOAD(std::string, ExampleVirt2, get_name, );
+        }
+    };
+    py::class_<ExampleVirt2, PyExampleVirt2>(m, "ExampleVirt2")
+        .def(py::init())
+        .def("get_name", &ExampleVirt2::get_name);
+    m.def("example_virt2_get_name",
+        [](const ExampleVirt2& obj) { return obj.get_name(); });
 }
 
 template <int N> class BreaksBase { public:

tests/test_class.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 import pytest
+import weakref
 
 import env  # noqa: F401
 
@@ -464,3 +465,33 @@ class ClassScope:
         m.register_duplicate_nested_class_type(ClassScope)
     expected = 'generic_type: type "YetAnotherDuplicateNested" is already registered!'
     assert str(exc_info.value) == expected
+
+
+def test_drake_11424():
+    # Define a derived class which *does not* overload the method.
+    # WARNING: The reproduction of this failure may be platform-specific, and
+    # seems to depend on the order of definition and/or the name of the classes
+    # defined. For example, trying to place this and the C++ code in
+    # `test_virtual_functions` makes `assert id_1 == id_2` below fail.
+    class Child1(m.ExampleVirt2): pass
+
+    id_1 = id(Child1)
+    assert m.example_virt2_get_name(m.ExampleVirt2()) == "ExampleVirt2"
+    assert m.example_virt2_get_name(Child1()) == "ExampleVirt2"
+
+    # Now delete everything (and ensure it's deleted).
+    wref = weakref.ref(Child1)
+    del Child1
+    pytest.gc_collect()
+    assert wref() == None
+
+    # Define a derived class which *does* define an overload.
+    class Child2(m.ExampleVirt2):
+        def get_name(self): return "Child2"
+
+    id_2 = id(Child2)
+    # assert id_1 == id_2  # This happens in CPython; not sure about PyPy.
+    assert m.example_virt2_get_name(m.ExampleVirt2()) == "ExampleVirt2"
+    # THIS WILL FAIL: This is using the cached `ExampleVirt2.get_name`, rather
+    # than re-inspect the Python dictionary.
+    assert m.example_virt2_get_name(Child2()) == "Child2"