Copying files as-is from pybind/pybind11#2834

The PR will be closed. Using this repo as attic.

Original git log:

```
commit 7fe7c96ad47f1e289f23a779566ec9ad3207a0af
Author: Ralf W. Grosse-Kunstleve <[email protected]>
Date:   Mon Mar 8 11:38:40 2021 -0800

    Tracing CpCtor, MvCtor, removing smart-pointer casters, renaming, more comments.

commit 64abe68d7236725825a545f15891e8ee683e45a9
Author: Ralf W. Grosse-Kunstleve <[email protected]>
Date:   Fri Jan 29 14:00:26 2021 -0800

    Adding comment, simplifying naming, cmake addition.

commit 7c1471ea612ed9a72c6559ba515c86ba6255f28f
Author: Ralf W. Grosse-Kunstleve <[email protected]>
Date:   Fri Jan 29 13:42:52 2021 -0800

    Copying files as-is from branch test_unique_ptr_member (PR #2672).
```

test_type_caster_bare_interface.cpp

@@ -0,0 +1,128 @@
+// Copied as-is from https://github.com/pybind/pybind11/pull/2834
+
+// Systematically exercises the detail::type_caster<> interface. This is going a step in the
+// direction of an integration test, to ensure multiple components of pybind11 work together
+// correctly. It is also useful to show the type_caster<> interface virtually clutter-free.
+
+// The entire type_caster load logic is intentionally omitted. The only purpose of this test is to
+// trace the behavior of the `static handle cast()` functions and the type_caster `operator`s.
+// Variable names are intentionally terse, to not distract from the more important function
+// signatures: valu(e), ref(erence), ptr (pointer), r = rvalue, m = mutable, c = const.
+
+#include "pybind11_tests.h"
+
+#include <string>
+
+namespace pybind11_tests {
+namespace type_caster_bare_interface {
+
+struct atyp { // Short for "any type".
+    std::string trace;
+    atyp() : trace("default") {}
+    atyp(atyp const &other) { trace = other.trace + "_CpCtor"; }
+    atyp(atyp &&other) { trace = other.trace + "_MvCtor"; }
+};
+
+// clang-format off
+
+atyp        rtrn_valu() { static atyp obj; obj.trace = "valu"; return obj; }
+atyp&&      rtrn_rref() { static atyp obj; obj.trace = "rref"; return std::move(obj); }
+atyp const& rtrn_cref() { static atyp obj; obj.trace = "cref"; return obj; }
+atyp&       rtrn_mref() { static atyp obj; obj.trace = "mref"; return obj; }
+atyp const* rtrn_cptr() { static atyp obj; obj.trace = "cptr"; return &obj; }
+atyp*       rtrn_mptr() { static atyp obj; obj.trace = "mptr"; return &obj; }
+
+std::string pass_valu(atyp obj)        { return "pass_valu:" + obj.trace; }
+std::string pass_rref(atyp&& obj)      { return "pass_rref:" + obj.trace; }
+std::string pass_cref(atyp const& obj) { return "pass_cref:" + obj.trace; }
+std::string pass_mref(atyp& obj)       { return "pass_mref:" + obj.trace; }
+std::string pass_cptr(atyp const* obj) { return "pass_cptr:" + obj->trace; }
+std::string pass_mptr(atyp* obj)       { return "pass_mptr:" + obj->trace; }
+
+// clang-format on
+
+} // namespace type_caster_bare_interface
+} // namespace pybind11_tests
+
+namespace pybind11 {
+namespace detail {
+
+using namespace pybind11_tests::type_caster_bare_interface;
+
+template <>
+struct type_caster<atyp> {
+    static constexpr auto name = _<atyp>();
+
+    // static handle cast(atyp, ...)
+    // is redundant (leads to ambiguous overloads).
+
+    static handle cast(atyp &&src, return_value_policy /*policy*/, handle /*parent*/) {
+        return str("cast_rref:" + src.trace).release();
+    }
+
+    static handle cast(atyp const &src, return_value_policy /*policy*/, handle /*parent*/) {
+        return str("cast_cref:" + src.trace).release();
+    }
+
+    static handle cast(atyp &src, return_value_policy /*policy*/, handle /*parent*/) {
+        return str("cast_mref:" + src.trace).release();
+    }
+
+    static handle cast(atyp const *src, return_value_policy /*policy*/, handle /*parent*/) {
+        return str("cast_cptr:" + src->trace).release();
+    }
+
+    static handle cast(atyp *src, return_value_policy /*policy*/, handle /*parent*/) {
+        return str("cast_mptr:" + src->trace).release();
+    }
+
+    template <typename T_>
+    using cast_op_type = conditional_t<
+        std::is_same<remove_reference_t<T_>, atyp const *>::value,
+        atyp const *,
+        conditional_t<
+            std::is_same<remove_reference_t<T_>, atyp *>::value,
+            atyp *,
+            conditional_t<
+                std::is_same<T_, atyp const &>::value,
+                atyp const &,
+                conditional_t<std::is_same<T_, atyp &>::value,
+                              atyp &,
+                              conditional_t<std::is_same<T_, atyp &&>::value, atyp &&, atyp>>>>>;
+
+    // clang-format off
+
+    operator atyp()        { static atyp obj; obj.trace = "valu"; return obj; }
+    operator atyp&&()      { static atyp obj; obj.trace = "rref"; return std::move(obj); }
+    operator atyp const&() { static atyp obj; obj.trace = "cref"; return obj; }
+    operator atyp&()       { static atyp obj; obj.trace = "mref"; return obj; }
+    operator atyp const*() { static atyp obj; obj.trace = "cptr"; return &obj; }
+    operator atyp*()       { static atyp obj; obj.trace = "mptr"; return &obj; }
+
+    // clang-format on
+
+    // The entire load logic is intentionally omitted.
+    bool load(handle /*src*/, bool /*convert*/) { return true; }
+};
+
+} // namespace detail
+} // namespace pybind11
+
+TEST_SUBMODULE(type_caster_bare_interface, m) {
+    namespace py = pybind11;
+    using namespace pybind11_tests::type_caster_bare_interface;
+
+    m.def("rtrn_valu", rtrn_valu);
+    m.def("rtrn_rref", rtrn_rref);
+    m.def("rtrn_cref", rtrn_cref);
+    m.def("rtrn_mref", rtrn_mref);
+    m.def("rtrn_cptr", rtrn_cptr);
+    m.def("rtrn_mptr", rtrn_mptr);
+
+    m.def("pass_valu", pass_valu);
+    m.def("pass_rref", pass_rref);
+    m.def("pass_cref", pass_cref);
+    m.def("pass_mref", pass_mref);
+    m.def("pass_cptr", pass_cptr);
+    m.def("pass_mptr", pass_mptr);
+}

test_type_caster_bare_interface.py

@@ -0,0 +1,37 @@
+# -*- coding: utf-8 -*-
+# Copied as-is from https://github.com/pybind/pybind11/pull/2834
+import re
+
+import pytest
+
+from pybind11_tests import type_caster_bare_interface as m
+
+
+@pytest.mark.parametrize(
+    "rtrn_f, expected",
+    [
+        (m.rtrn_valu, "cast_rref:valu_CpCtor"),
+        (m.rtrn_rref, "cast_rref:rref"),
+        (m.rtrn_cref, "cast_cref:cref"),
+        (m.rtrn_mref, "cast_mref:mref"),
+        (m.rtrn_cptr, "cast_cptr:cptr"),
+        (m.rtrn_mptr, "cast_mptr:mptr"),
+    ],
+)
+def test_cast(rtrn_f, expected):
+    assert re.match(expected, rtrn_f())
+
+
+@pytest.mark.parametrize(
+    "pass_f, expected",
+    [
+        (m.pass_valu, "pass_valu:rref_MvCtor"),
+        (m.pass_rref, "pass_rref:rref"),
+        (m.pass_cref, "pass_cref:cref"),
+        (m.pass_mref, "pass_mref:mref"),
+        (m.pass_cptr, "pass_cptr:cptr"),
+        (m.pass_mptr, "pass_mptr:mptr"),
+    ],
+)
+def test_operator(pass_f, expected):
+    assert re.match(expected, pass_f(None))