[smart_holder] Unique ptr deleter roundtrip tests and fix (#4921)

* Roundtrip through unique pointer with custom deleter.

Currently failing.

* Ensure the custom deleter is copied back to the unique pointer.

Feels like there's still a gap around the raw pointer flavour, but this at least
makes the unit test of the previous commit succeed.

* Add deleter roundtrip for const atyp.

Currently failing, custom deleter is lost.

* Fix storing deleter for const unique ptr.

Unit test from the previous commit passes.

* Remove SFINEA deleter assignment.

At the construction of the smart holder, it is either a del_fun, or a default constructed deleter, so this complexity is unnecessary.

* Clang format.

* Fixes for ci.

Clang 3.6 requires the extra constructors in the custom_deleter.

* fix(smart_holder): Loosen requirement on deleter to be default constructible.

And some other PR feedback.

* fix(smart_holder): Custom deleter in unit tests traces constructions.

* fix(smart_holder): Use pybind11_fail instead of assert.

* fix(smart_holder): Add unit tests for the default constructible deleter.

* fix(smart_holder): Use regex matching for deleter constructors in unit tests.

tests/test_class_sh_basic.cpp

@@ -28,6 +28,43 @@ struct uconsumer { // unique_ptr consumer
     const std::unique_ptr<atyp> &rtrn_cref() const { return held; }
 };
 
+/// Custom deleter that is default constructible.
+struct custom_deleter {
+    std::string trace_txt;
+
+    custom_deleter() = default;
+    explicit custom_deleter(const std::string &trace_txt_) : trace_txt(trace_txt_) {}
+
+    custom_deleter(const custom_deleter &other) { trace_txt = other.trace_txt + "_CpCtor"; }
+
+    custom_deleter &operator=(const custom_deleter &rhs) {
+        trace_txt = rhs.trace_txt + "_CpLhs";
+        return *this;
+    }
+
+    custom_deleter(custom_deleter &&other) noexcept {
+        trace_txt = other.trace_txt + "_MvCtorTo";
+        other.trace_txt += "_MvCtorFrom";
+    }
+
+    custom_deleter &operator=(custom_deleter &&rhs) noexcept {
+        trace_txt = rhs.trace_txt + "_MvLhs";
+        rhs.trace_txt += "_MvRhs";
+        return *this;
+    }
+
+    void operator()(atyp *p) const { std::default_delete<atyp>()(p); }
+    void operator()(const atyp *p) const { std::default_delete<const atyp>()(p); }
+};
+static_assert(std::is_default_constructible<custom_deleter>::value, "");
+
+/// Custom deleter that is not default constructible.
+struct custom_deleter_nd : custom_deleter {
+    custom_deleter_nd() = delete;
+    explicit custom_deleter_nd(const std::string &trace_txt_) : custom_deleter(trace_txt_) {}
+};
+static_assert(!std::is_default_constructible<custom_deleter_nd>::value, "");
+
 // clang-format off
 
 atyp        rtrn_valu() { atyp obj{"rtrn_valu"}; return obj; }
@@ -64,6 +101,18 @@ std::unique_ptr<atyp const, sddc> rtrn_udcp() { return std::unique_ptr<atyp cons
 std::string pass_udmp(std::unique_ptr<atyp,       sddm> obj) { return "pass_udmp:" + obj->mtxt; }
 std::string pass_udcp(std::unique_ptr<atyp const, sddc> obj) { return "pass_udcp:" + obj->mtxt; }
 
+std::unique_ptr<atyp,       custom_deleter> rtrn_udmp_del() { return std::unique_ptr<atyp,       custom_deleter>(new atyp{"rtrn_udmp_del"}, custom_deleter{"udmp_deleter"}); }
+std::unique_ptr<atyp const, custom_deleter> rtrn_udcp_del() { return std::unique_ptr<atyp const, custom_deleter>(new atyp{"rtrn_udcp_del"}, custom_deleter{"udcp_deleter"}); }
+
+std::string pass_udmp_del(std::unique_ptr<atyp,       custom_deleter> obj) { return "pass_udmp_del:" + obj->mtxt + "," + obj.get_deleter().trace_txt; }
+std::string pass_udcp_del(std::unique_ptr<atyp const, custom_deleter> obj) { return "pass_udcp_del:" + obj->mtxt + "," + obj.get_deleter().trace_txt; }
+
+std::unique_ptr<atyp,       custom_deleter_nd> rtrn_udmp_del_nd() { return std::unique_ptr<atyp,       custom_deleter_nd>(new atyp{"rtrn_udmp_del_nd"}, custom_deleter_nd{"udmp_deleter_nd"}); }
+std::unique_ptr<atyp const, custom_deleter_nd> rtrn_udcp_del_nd() { return std::unique_ptr<atyp const, custom_deleter_nd>(new atyp{"rtrn_udcp_del_nd"}, custom_deleter_nd{"udcp_deleter_nd"}); }
+
+std::string pass_udmp_del_nd(std::unique_ptr<atyp,       custom_deleter_nd> obj) { return "pass_udmp_del_nd:" + obj->mtxt + "," + obj.get_deleter().trace_txt; }
+std::string pass_udcp_del_nd(std::unique_ptr<atyp const, custom_deleter_nd> obj) { return "pass_udcp_del_nd:" + obj->mtxt + "," + obj.get_deleter().trace_txt; }
+
 // clang-format on
 
 // Helpers for testing.
@@ -130,6 +179,18 @@ TEST_SUBMODULE(class_sh_basic, m) {
     m.def("pass_udmp", pass_udmp);
     m.def("pass_udcp", pass_udcp);
 
+    m.def("rtrn_udmp_del", rtrn_udmp_del);
+    m.def("rtrn_udcp_del", rtrn_udcp_del);
+
+    m.def("pass_udmp_del", pass_udmp_del);
+    m.def("pass_udcp_del", pass_udcp_del);
+
+    m.def("rtrn_udmp_del_nd", rtrn_udmp_del_nd);
+    m.def("rtrn_udcp_del_nd", rtrn_udcp_del_nd);
+
+    m.def("pass_udmp_del_nd", pass_udmp_del_nd);
+    m.def("pass_udcp_del_nd", pass_udcp_del_nd);
+
     py::classh<uconsumer>(m, "uconsumer")
         .def(py::init<>())
         .def("valid", &uconsumer::valid)