[Experiment] std::type_index & unnamed namespace

Related to PRs pybind#4313, pybind#4315

include/pybind11/detail/internals.h

@@ -209,6 +209,8 @@ struct internals {
         PYBIND11_TLS_FREE(tstate);
     }
 #endif
+
+    std::unordered_map<std::type_index, std::vector<std::string>> std_type_index_registry;
 };
 
 /// Additional type information which does not fit into the PyTypeObject.

tests/CMakeLists.txt

@@ -156,6 +156,8 @@ set(PYBIND11_TEST_FILES
     test_tagbased_polymorphic
     test_thread
     test_union
+    test_unnamed_namespace_a
+    test_unnamed_namespace_b
     test_virtual_functions)
 
 # Invoking cmake with something like:

tests/test_unnamed_namespace_a.cpp

@@ -0,0 +1,23 @@
+#include <pybind11/stl.h>
+
+#include "pybind11_tests.h"
+
+namespace {
+struct any_struct {};
+} // namespace
+
+TEST_SUBMODULE(unnamed_namespace_a, m) {
+    m.attr("name") = "A";
+
+    py::detail::get_internals()
+        .std_type_index_registry[std::type_index(typeid(any_struct))]
+        .push_back("A");
+
+    m.def("std_type_index_registry_dump", []() {
+        py::list items;
+        for (const auto &it : py::detail::get_internals().std_type_index_registry) {
+            items.append(py::make_tuple(it.first.name(), it.second));
+        }
+        return items;
+    });
+}

tests/test_unnamed_namespace_a.py

@@ -0,0 +1,15 @@
+import pytest
+
+from pybind11_tests import unnamed_namespace_a as m
+
+
+def test_inspect():
+    assert m.name == "A"
+    reg = m.std_type_index_registry_dump()
+    if len(reg) == 1:
+        assert tuple(sorted(reg[0][1])) == ("A", "B")
+        pytest.skip("std::type_index-EQ-BAD")
+    if len(reg) == 2:
+        assert tuple(sorted([reg[0][1][0], reg[1][1][0]])) == ("A", "B")
+        pytest.skip("std::type_index-NE-GOOD")
+    assert reg is None  # Sure to fail.

tests/test_unnamed_namespace_b.cpp

@@ -0,0 +1,13 @@
+#include "pybind11_tests.h"
+
+namespace {
+struct any_struct {};
+} // namespace
+
+TEST_SUBMODULE(unnamed_namespace_b, m) {
+    m.attr("name") = "B";
+
+    py::detail::get_internals()
+        .std_type_index_registry[std::type_index(typeid(any_struct))]
+        .push_back("B");
+}

tests/test_unnamed_namespace_b.py

@@ -0,0 +1,5 @@
+from pybind11_tests import unnamed_namespace_b as m
+
+
+def test_inspect():
+    assert m.name == "B"