example no intermediate type downcast. show downcast hurts identity

tmp/BUILD.bazel

@@ -0,0 +1,31 @@
+load(
+    "//tools/skylark:drake_py.bzl",
+    "drake_py_binary",
+    "drake_py_library",
+)
+load(
+    "//tools/skylark:pybind.bzl",
+    "pybind_py_library",
+)
+
+drake_py_library(
+    name = "module_py",
+    srcs = ["__init__.py"],
+)
+
+pybind_py_library(
+    name = "cc_py",
+    cc_deps = ["//:drake_shared_library"],
+    cc_so_name = "cc",
+    cc_srcs = ["cc_py.cc"],
+    py_deps = [
+        ":module_py",
+        "@drake//bindings/pydrake",
+    ],
+)
+
+drake_py_binary(
+    name = "repro",
+    srcs = ["repro.py"],
+    deps = [":cc_py"],
+)

tmp/__init__.py

@@ -0,0 +1 @@
+# empty.

tmp/cc_py.cc

@@ -0,0 +1,39 @@
+#include <pybind11/pybind11.h>
+
+#include "drake/common/drake_assert.h"
+#include "drake/systems/framework/diagram_builder.h"
+#include "drake/systems/primitives/adder.h"
+
+namespace py = pybind11;
+
+namespace drake {
+namespace pydrake {
+
+class MyDiagram : public systems::Diagram<double> {
+ public:
+  MyDiagram() {
+    systems::DiagramBuilder<double> builder;
+    builder.AddSystem<systems::Adder<double>>(0, 1);
+    builder.BuildInto(this);
+  }
+};
+
+PYBIND11_MODULE(cc, m) {
+  py::module::import("pydrake.systems.framework");
+
+  m.def("make_my_diagram_as_system", []()
+      -> std::unique_ptr<systems::System<double>> {
+    return std::make_unique<MyDiagram>();
+  });
+  m.def("make_my_diagram_as_diagram", []()
+      -> std::unique_ptr<systems::Diagram<double>> {
+    return std::make_unique<MyDiagram>();
+  });
+
+  m.def("as_diagram", [](systems::System<double>* system) {
+    return dynamic_cast<systems::Diagram<double>*>(system);
+  }, py::return_value_policy::reference, py::keep_alive<0, 1>());
+}
+
+}  // namespace
+}  // namespace pydrake

tmp/repro.py

@@ -0,0 +1,9 @@
+from pydrake.systems.framework import LeafSystem, Diagram
+from drake.tmp.cc import make_my_diagram_as_system, make_my_diagram_as_diagram, as_diagram
+
+system = make_my_diagram_as_system()
+print(system)
+print(as_diagram(system))
+assert as_diagram(system) is not system  # :(
+diagram = make_my_diagram_as_diagram()
+print(diagram)