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

docs/classes.rst

@@ -58,6 +58,16 @@ 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 on many platforms, but not all. The `XFAIL_CONDITION` in
+    tests/test_unnamed_namespace_a.py encodes the currently known conditions.
+    For background see `#4319 <https://github.com/pybind/pybind11/pull/4319>`_.
+    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

@@ -123,7 +123,8 @@ 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 (PYBIND11_INTERNALS_VERSION <= 4 && defined(__GLIBCXX__))                                     \
+    || (PYBIND11_INTERNALS_VERSION >= 5 && !defined(_LIBCPP_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

@@ -155,6 +155,8 @@ 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 +1,38 @@
+#include "pybind11_tests.h"
+
+namespace {
+struct any_struct {};
+} // namespace
+
+TEST_SUBMODULE(unnamed_namespace_a, m) {
+    if (py::detail::get_type_info(typeid(any_struct)) == nullptr) {
+        py::class_<any_struct>(m, "unnamed_namespace_a_any_struct");
+    } else {
+        m.attr("unnamed_namespace_a_any_struct") = py::none();
+    }
+    m.attr("PYBIND11_INTERNALS_VERSION") = PYBIND11_INTERNALS_VERSION;
+    m.attr("defined_WIN32_or__WIN32") =
+#if defined(WIN32) || defined(_WIN32)
+        true;
+#else
+        false;
+#endif
+    m.attr("defined___clang__") =
+#if defined(__clang__)
+        true;
+#else
+        false;
+#endif
+    m.attr("defined__LIBCPP_VERSION") =
+#if defined(_LIBCPP_VERSION)
+        true;
+#else
+        false;
+#endif
+    m.attr("defined___GLIBCXX__") =
+#if defined(__GLIBCXX__)
+        true;
+#else
+        false;
+#endif
+}

tests/test_unnamed_namespace_a.py

@@ -0,0 +1,34 @@
+import pytest
+
+from pybind11_tests import unnamed_namespace_a as m
+from pybind11_tests import unnamed_namespace_b as mb
+
+XFAIL_CONDITION = (
+    "(m.PYBIND11_INTERNALS_VERSION <= 4 and (m.defined___clang__ or not m.defined___GLIBCXX__))"
+    " or "
+    "(m.PYBIND11_INTERNALS_VERSION >= 5 and not m.defined_WIN32_or__WIN32"
+    " and "
+    "(m.defined___clang__ or m.defined__LIBCPP_VERSION))"
+)
+XFAIL_REASON = "Known issues: https://github.com/pybind/pybind11/pull/4319"
+
+
+@pytest.mark.xfail(XFAIL_CONDITION, reason=XFAIL_REASON, strict=False)
+@pytest.mark.parametrize(
+    "any_struct", [m.unnamed_namespace_a_any_struct, mb.unnamed_namespace_b_any_struct]
+)
+def test_have_class_any_struct(any_struct):
+    assert any_struct is not None
+
+
+def test_have_at_least_one_class_any_struct():
+    assert (
+        m.unnamed_namespace_a_any_struct is not None
+        or mb.unnamed_namespace_b_any_struct is not None
+    )
+
+
+@pytest.mark.xfail(XFAIL_CONDITION, reason=XFAIL_REASON, strict=True)
+def test_have_both_class_any_struct():
+    assert m.unnamed_namespace_a_any_struct is not None
+    assert mb.unnamed_namespace_b_any_struct is not None

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) {
+    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();
+    }
+}

tests/test_unnamed_namespace_b.py

@@ -0,0 +1,5 @@
+from pybind11_tests import unnamed_namespace_b as m
+
+
+def test_have_attr_any_struct():
+    assert hasattr(m, "unnamed_namespace_b_any_struct")