Use std::hash<std::type_index>, std::equal_to<std::type_index> everywhere **except when libc++ is in use**

docs/classes.rst

@@ -58,6 +58,15 @@ interactive Python session demonstrating this example is shown below:
     Static member functions can be bound in the same way using
     :func:`class_::def_static`.
 
+.. note::
+
+    Binding C++ types in unnamed namespaces (also known as anonymous namespaces)
+    works reliably only with GCC and MSVC, but not with CLANG.
+    See `#4316 <https://github.com/pybind/pybind11/pull/4316>`_ for background.
+    If portability is a concern, it is therefore not recommended to bind C++
+    types in unnamed namespaces. It will be safest to manually pick unique
+    namespace names.
+
 Keyword and default arguments
 =============================
 It is possible to specify keyword and default arguments using the syntax

include/pybind11/detail/internals.h

@@ -34,7 +34,7 @@
 /// further ABI-incompatible changes may be made before the ABI is officially
 /// changed to the new version.
 #ifndef PYBIND11_INTERNALS_VERSION
-#    define PYBIND11_INTERNALS_VERSION 4
+#    define PYBIND11_INTERNALS_VERSION 5
 #endif
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
@@ -114,7 +114,7 @@ inline void tls_replace_value(PYBIND11_TLS_KEY_REF key, void *value) {
 // libstdc++, this doesn't happen: equality and the type_index hash are based on the type name,
 // which works.  If not under a known-good stl, provide our own name-based hash and equality
 // functions that use the type name.
-#if defined(__GLIBCXX__)
+#if !defined(__apple_build_version__)
 inline bool same_type(const std::type_info &lhs, const std::type_info &rhs) { return lhs == rhs; }
 using type_hash = std::hash<std::type_index>;
 using type_equal_to = std::equal_to<std::type_index>;

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 +1,9 @@
+#include "pybind11_tests.h"
+
+namespace {
+struct any_struct {};
+} // namespace
+
+TEST_SUBMODULE(unnamed_namespace_a, m) {
+    py::class_<any_struct>(m, "unnamed_namespace_a_any_struct");
+}

tests/test_unnamed_namespace_a.py

@@ -0,0 +1,8 @@
+# NOTE: This test relies on pytest SORT ORDER:
+#       test_unnamed_namespace_a.py imported before test_unnamed_namespace_b.py
+
+from pybind11_tests import unnamed_namespace_a as m
+
+
+def test_have_class_any_struct():
+    assert m.unnamed_namespace_a_any_struct is not None

tests/test_unnamed_namespace_b.cpp

@@ -0,0 +1,19 @@
+#include "pybind11_tests.h"
+
+namespace {
+struct any_struct {};
+} // namespace
+
+TEST_SUBMODULE(unnamed_namespace_b, m) {
+    if (py::detail::get_type_info(typeid(any_struct)) == nullptr) {
+        py::class_<any_struct>(m, "unnamed_namespace_b_any_struct");
+    } else {
+        m.attr("unnamed_namespace_b_any_struct") = py::none();
+    }
+    m.attr("defined___clang__") =
+#if defined(__clang__)
+        true;
+#else
+        false;
+#endif
+}

tests/test_unnamed_namespace_b.py

@@ -0,0 +1,15 @@
+# NOTE: This test relies on pytest SORT ORDER:
+#       test_unnamed_namespace_a.py imported before test_unnamed_namespace_b.py
+
+import pytest
+
+from pybind11_tests import unnamed_namespace_b as m
+
+
+@pytest.mark.xfail(
+    "m.defined___clang__",
+    reason="Known issue with all clang versions: https://github.com/pybind/pybind11/pull/4316",
+    strict=True,
+)
+def test_have_class_any_struct():
+    assert m.unnamed_namespace_b_any_struct is not None