Copying tests as-is from xxx_value_ptr_xxx_holder branch.

https://github.com/rwgk/pybind11/tree/xxx_value_ptr_xxx_holder

Systematically exercising returning and passing unique_ptr<T>, shared_ptr<T>
with unique_ptr, shared_ptr holder.

Observations:

test_holder_unique_ptr:
  make_unique_pointee  OK
  pass_unique_pointee  BUILD_FAIL (as documented)
  make_shared_pointee  Abort free(): double free detected
  pass_shared_pointee  RuntimeError: Unable to load a custom holder type from a default-holder instance

test_holder_shared_ptr:
  make_unique_pointee  Segmentation fault (#1138)
  pass_unique_pointee  BUILD_FAIL (as documented)
  make_shared_pointee  OK
  pass_shared_pointee  OK

tests/test_holder_shared_ptr.cpp

@@ -0,0 +1,69 @@
+// KEEP IN SYNC WITH test_holder_unique_ptr.cpp
+
+#include "pybind11_tests.h"
+
+#include <iostream>
+#include <memory>
+
+namespace pybind11_tests {
+namespace holder_shared_ptr {
+
+inline void to_cout(std::string text) { std::cout << text << std::endl; }
+
+class pointee { // NOT copyable.
+  public:
+    pointee() { to_cout("pointee::pointee()"); }
+
+    int get_int() const {
+        to_cout("pointee::get_int()");
+        return 213;
+    }
+
+    ~pointee() { to_cout("~pointee()"); }
+
+  private:
+    pointee(const pointee &) = delete;
+    pointee(pointee &&) = delete;
+    pointee &operator=(const pointee &) = delete;
+    pointee &operator=(pointee &&) = delete;
+};
+
+inline std::unique_ptr<pointee> make_unique_pointee() {
+    return std::unique_ptr<pointee>(new pointee);
+}
+
+inline std::shared_ptr<pointee> make_shared_pointee() {
+    return std::unique_ptr<pointee>(new pointee);
+}
+
+inline int pass_unique_pointee(std::unique_ptr<pointee> ptr) {
+    return 4000 + ptr->get_int();
+}
+
+inline int pass_shared_pointee(std::shared_ptr<pointee> ptr) {
+    return 5000 + ptr->get_int();
+}
+
+inline pointee* get_static_pointee() {
+  static pointee cpp_instance;
+  return &cpp_instance;
+}
+
+TEST_SUBMODULE(holder_shared_ptr, m) {
+    m.def("to_cout", to_cout);
+
+    py::class_<pointee, std::shared_ptr<pointee>>(m, "pointee")
+        .def(py::init<>())
+        .def("get_int", &pointee::get_int);
+
+    m.def("make_unique_pointee", make_unique_pointee);
+    m.def("make_shared_pointee", make_shared_pointee);
+    // m.def("pass_unique_pointee", pass_unique_pointee);
+    m.def("pass_shared_pointee", pass_shared_pointee);
+
+    m.def("get_static_pointee",
+          get_static_pointee, py::return_value_policy::reference);
+}
+
+} // namespace holder_shared_ptr
+} // namespace pybind11_tests

tests/test_holder_shared_ptr.py

@@ -0,0 +1,56 @@
+# -*- coding: utf-8 -*-
+# KEEP IN SYNC WITH test_holder_unique_ptr.py
+import pytest
+
+from pybind11_tests import holder_shared_ptr as m
+
+
+def test_make_unique_pointee():
+    m.to_cout("")
+    m.to_cout("")
+    m.to_cout("make_unique_pointee")
+    obj = m.make_unique_pointee()
+    assert obj.get_int() == 213
+    m.to_cout("")
+
+
+def test_make_shared_pointee():
+    m.to_cout("")
+    m.to_cout("")
+    m.to_cout("make_shared_pointee")
+    obj = m.make_shared_pointee()
+    assert obj.get_int() == 213
+    m.to_cout("")
+
+
+def test_pass_unique_pointee():
+    m.to_cout("")
+    m.to_cout("")
+    m.to_cout("pass_unique_pointee")
+    obj = m.make_shared_pointee()
+    assert obj.get_int() == 213
+    i = m.pass_unique_pointee(obj)
+    assert i == 4213
+    m.to_cout("")
+
+
+def test_pass_shared_pointee():
+    m.to_cout("")
+    m.to_cout("")
+    m.to_cout("pass_shared_pointee")
+    obj = m.make_shared_pointee()
+    assert obj.get_int() == 213
+    i = m.pass_shared_pointee(obj)
+    assert i == 5213
+    m.to_cout("")
+
+
+def test_get_static_pointee():
+    m.to_cout("")
+    m.to_cout("")
+    m.to_cout("get_static_pointee")
+    obj = m.get_static_pointee()
+    assert obj.get_int() == 213
+    with pytest.raises(RuntimeError) as excinfo:
+        m.pass_shared_pointee(obj)
+    assert "Unable to cast from non-held to held instance" in str(excinfo.value)

tests/test_holder_unique_ptr.cpp

@@ -0,0 +1,69 @@
+// KEEP IN SYNC WITH test_holder_shared_ptr.cpp
+
+#include "pybind11_tests.h"
+
+#include <iostream>
+#include <memory>
+
+namespace pybind11_tests {
+namespace holder_unique_ptr {
+
+inline void to_cout(std::string text) { std::cout << text << std::endl; }
+
+class pointee { // NOT copyable.
+  public:
+    pointee() { to_cout("pointee::pointee()"); }
+
+    int get_int() const {
+        to_cout("pointee::get_int()");
+        return 213;
+    }
+
+    ~pointee() { to_cout("~pointee()"); }
+
+  private:
+    pointee(const pointee &) = delete;
+    pointee(pointee &&) = delete;
+    pointee &operator=(const pointee &) = delete;
+    pointee &operator=(pointee &&) = delete;
+};
+
+inline std::unique_ptr<pointee> make_unique_pointee() {
+    return std::unique_ptr<pointee>(new pointee);
+}
+
+inline std::shared_ptr<pointee> make_shared_pointee() {
+    return std::unique_ptr<pointee>(new pointee);
+}
+
+inline int pass_unique_pointee(std::unique_ptr<pointee> ptr) {
+    return 4000 + ptr->get_int();
+}
+
+inline int pass_shared_pointee(std::shared_ptr<pointee> ptr) {
+    return 5000 + ptr->get_int();
+}
+
+inline pointee* get_static_pointee() {
+  static pointee cpp_instance;
+  return &cpp_instance;
+}
+
+TEST_SUBMODULE(holder_unique_ptr, m) {
+    m.def("to_cout", to_cout);
+
+    py::class_<pointee>(m, "pointee")
+        .def(py::init<>())
+        .def("get_int", &pointee::get_int);
+
+    m.def("make_unique_pointee", make_unique_pointee);
+    m.def("make_shared_pointee", make_shared_pointee);
+    // m.def("pass_unique_pointee", pass_unique_pointee);
+    m.def("pass_shared_pointee", pass_shared_pointee);
+
+    m.def("get_static_pointee",
+          get_static_pointee, py::return_value_policy::reference);
+}
+
+} // namespace holder_unique_ptr
+} // namespace pybind11_tests

tests/test_holder_unique_ptr.py

@@ -0,0 +1,56 @@
+# -*- coding: utf-8 -*-
+# KEEP IN SYNC WITH test_holder_shared_ptr.py
+import pytest
+
+from pybind11_tests import holder_unique_ptr as m
+
+
+def test_make_unique_pointee():
+    m.to_cout("")
+    m.to_cout("")
+    m.to_cout("make_unique_pointee")
+    obj = m.make_unique_pointee()
+    assert obj.get_int() == 213
+    m.to_cout("")
+
+
+def test_make_shared_pointee():
+    m.to_cout("")
+    m.to_cout("")
+    m.to_cout("make_shared_pointee")
+    obj = m.make_shared_pointee()
+    assert obj.get_int() == 213
+    m.to_cout("")
+
+
+def test_pass_unique_pointee():
+    m.to_cout("")
+    m.to_cout("")
+    m.to_cout("pass_unique_pointee")
+    obj = m.make_unique_pointee()
+    assert obj.get_int() == 213
+    i = m.pass_unique_pointee(obj)
+    assert i == 4213
+    m.to_cout("")
+
+
+def test_pass_shared_pointee():
+    m.to_cout("")
+    m.to_cout("")
+    m.to_cout("pass_shared_pointee")
+    obj = m.make_unique_pointee()
+    assert obj.get_int() == 213
+    i = m.pass_shared_pointee(obj)
+    assert i == 5213
+    m.to_cout("")
+
+
+def test_get_static_pointee():
+    m.to_cout("")
+    m.to_cout("")
+    m.to_cout("get_static_pointee")
+    obj = m.get_static_pointee()
+    assert obj.get_int() == 213
+    with pytest.raises(RuntimeError) as excinfo:
+        m.pass_unique_pointee(obj)
+    assert "Unable to cast from non-held to held instance" in str(excinfo.value)