From a06949a9265014b3c581396c4f37c45ccc03dea6 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Sun, 6 Nov 2022 15:05:15 -0800
Subject: [PATCH 01/24] Try using `std::hash<std::type_index>`,
 `std::equal_to<std::type_index>` everywhere.

From PR #4316 we know that types in the unnamed namespace in different translation units do not compare equal, as desired.

But do types in named namespaces compare equal, as desired?
---
 include/pybind11/detail/internals.h | 27 +--------------------------
 1 file changed, 1 insertion(+), 26 deletions(-)

diff --git a/include/pybind11/detail/internals.h b/include/pybind11/detail/internals.h
index 6fd61098c4..0064684212 100644
--- a/include/pybind11/detail/internals.h
+++ b/include/pybind11/detail/internals.h
@@ -114,35 +114,10 @@ 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__)
 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>;
-#else
-inline bool same_type(const std::type_info &lhs, const std::type_info &rhs) {
-    return lhs.name() == rhs.name() || std::strcmp(lhs.name(), rhs.name()) == 0;
-}
-
-struct type_hash {
-    size_t operator()(const std::type_index &t) const {
-        size_t hash = 5381;
-        const char *ptr = t.name();
-        while (auto c = static_cast<unsigned char>(*ptr++)) {
-            hash = (hash * 33) ^ c;
-        }
-        return hash;
-    }
-};
-
-struct type_equal_to {
-    bool operator()(const std::type_index &lhs, const std::type_index &rhs) const {
-        return lhs.name() == rhs.name() || std::strcmp(lhs.name(), rhs.name()) == 0;
-    }
-};
-#endif
 
 template <typename value_type>
-using type_map = std::unordered_map<std::type_index, value_type, type_hash, type_equal_to>;
+using type_map = std::unordered_map<std::type_index, value_type>;
 
 struct override_hash {
     inline size_t operator()(const std::pair<const PyObject *, const char *> &v) const {

From 9d70eba39a0105eddf3830d0cd9f430ed34dd2de Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Sun, 6 Nov 2022 21:22:40 -0800
Subject: [PATCH 02/24] Revert "Try using `std::hash<std::type_index>`,
 `std::equal_to<std::type_index>` everywhere."

This reverts commit a06949a9265014b3c581396c4f37c45ccc03dea6.
---
 include/pybind11/detail/internals.h | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/include/pybind11/detail/internals.h b/include/pybind11/detail/internals.h
index 0064684212..6fd61098c4 100644
--- a/include/pybind11/detail/internals.h
+++ b/include/pybind11/detail/internals.h
@@ -114,10 +114,35 @@ 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__)
 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>;
+#else
+inline bool same_type(const std::type_info &lhs, const std::type_info &rhs) {
+    return lhs.name() == rhs.name() || std::strcmp(lhs.name(), rhs.name()) == 0;
+}
+
+struct type_hash {
+    size_t operator()(const std::type_index &t) const {
+        size_t hash = 5381;
+        const char *ptr = t.name();
+        while (auto c = static_cast<unsigned char>(*ptr++)) {
+            hash = (hash * 33) ^ c;
+        }
+        return hash;
+    }
+};
+
+struct type_equal_to {
+    bool operator()(const std::type_index &lhs, const std::type_index &rhs) const {
+        return lhs.name() == rhs.name() || std::strcmp(lhs.name(), rhs.name()) == 0;
+    }
+};
+#endif
 
 template <typename value_type>
-using type_map = std::unordered_map<std::type_index, value_type>;
+using type_map = std::unordered_map<std::type_index, value_type, type_hash, type_equal_to>;
 
 struct override_hash {
     inline size_t operator()(const std::pair<const PyObject *, const char *> &v) const {

From 9141519166efa03d36337eafa202bc82afacc575 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Sun, 6 Nov 2022 21:27:29 -0800
Subject: [PATCH 03/24] Use "our own name-based hash and equality functions"
 for `std::type_index` only under macOS, based on results shown under
 https://github.com/pybind/pybind11/pull/4316#issuecomment-1305097879

---
 include/pybind11/detail/internals.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/pybind11/detail/internals.h b/include/pybind11/detail/internals.h
index 6fd61098c4..768a290fe2 100644
--- a/include/pybind11/detail/internals.h
+++ b/include/pybind11/detail/internals.h
@@ -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>;

From ba747dcfa4f525a47e70ac20695cc4f33f8b9f73 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Mon, 7 Nov 2022 03:12:47 -0800
Subject: [PATCH 04/24] Patch in PR #4313: Minimal reproducer for clash when
 binding types defined in the unnamed namespace.

---
 tests/CMakeLists.txt               | 2 ++
 tests/test_unnamed_namespace_a.cpp | 9 +++++++++
 tests/test_unnamed_namespace_a.py  | 5 +++++
 tests/test_unnamed_namespace_b.cpp | 9 +++++++++
 tests/test_unnamed_namespace_b.py  | 5 +++++
 5 files changed, 30 insertions(+)
 create mode 100644 tests/test_unnamed_namespace_a.cpp
 create mode 100644 tests/test_unnamed_namespace_a.py
 create mode 100644 tests/test_unnamed_namespace_b.cpp
 create mode 100644 tests/test_unnamed_namespace_b.py

diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 491f215cef..285f43b5b9 100644
--- a/tests/CMakeLists.txt
+++ b/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:
diff --git a/tests/test_unnamed_namespace_a.cpp b/tests/test_unnamed_namespace_a.cpp
new file mode 100644
index 0000000000..4f6472c386
--- /dev/null
+++ b/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");
+}
diff --git a/tests/test_unnamed_namespace_a.py b/tests/test_unnamed_namespace_a.py
new file mode 100644
index 0000000000..b823376a4b
--- /dev/null
+++ b/tests/test_unnamed_namespace_a.py
@@ -0,0 +1,5 @@
+from pybind11_tests import unnamed_namespace_a as m
+
+
+def test_have_type():
+    assert hasattr(m, "unnamed_namespace_a_any_struct")
diff --git a/tests/test_unnamed_namespace_b.cpp b/tests/test_unnamed_namespace_b.cpp
new file mode 100644
index 0000000000..8c9b140098
--- /dev/null
+++ b/tests/test_unnamed_namespace_b.cpp
@@ -0,0 +1,9 @@
+#include "pybind11_tests.h"
+
+namespace {
+struct any_struct {};
+} // namespace
+
+TEST_SUBMODULE(unnamed_namespace_b, m) {
+    py::class_<any_struct>(m, "unnamed_namespace_b_any_struct");
+}
diff --git a/tests/test_unnamed_namespace_b.py b/tests/test_unnamed_namespace_b.py
new file mode 100644
index 0000000000..0550bc4347
--- /dev/null
+++ b/tests/test_unnamed_namespace_b.py
@@ -0,0 +1,5 @@
+from pybind11_tests import unnamed_namespace_b as m
+
+
+def test_have_type():
+    assert hasattr(m, "unnamed_namespace_b_any_struct")

From 191267055bb9904e2b948a60736225e87af342b2 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Mon, 7 Nov 2022 04:13:28 -0800
Subject: [PATCH 05/24] test_unnamed_namespace_b xfail for clang

---
 tests/test_unnamed_namespace_a.py  |  7 +++++--
 tests/test_unnamed_namespace_b.cpp | 12 +++++++++++-
 tests/test_unnamed_namespace_b.py  | 14 ++++++++++++--
 3 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/tests/test_unnamed_namespace_a.py b/tests/test_unnamed_namespace_a.py
index b823376a4b..995c852e5f 100644
--- a/tests/test_unnamed_namespace_a.py
+++ b/tests/test_unnamed_namespace_a.py
@@ -1,5 +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_type():
-    assert hasattr(m, "unnamed_namespace_a_any_struct")
+def test_have_class_any_struct():
+    assert m.unnamed_namespace_a_any_struct is not None
diff --git a/tests/test_unnamed_namespace_b.cpp b/tests/test_unnamed_namespace_b.cpp
index 8c9b140098..87e5140950 100644
--- a/tests/test_unnamed_namespace_b.cpp
+++ b/tests/test_unnamed_namespace_b.cpp
@@ -5,5 +5,15 @@ struct any_struct {};
 } // namespace
 
 TEST_SUBMODULE(unnamed_namespace_b, m) {
-    py::class_<any_struct>(m, "unnamed_namespace_b_any_struct");
+    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
 }
diff --git a/tests/test_unnamed_namespace_b.py b/tests/test_unnamed_namespace_b.py
index 0550bc4347..cb4c6f412a 100644
--- a/tests/test_unnamed_namespace_b.py
+++ b/tests/test_unnamed_namespace_b.py
@@ -1,5 +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
 
 
-def test_have_type():
-    assert hasattr(m, "unnamed_namespace_b_any_struct")
+@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

From d8b71181d9bfd9a144c4667840532adb627b1af8 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Mon, 7 Nov 2022 04:30:28 -0800
Subject: [PATCH 06/24] `PYBIND11_INTERNALS_VERSION 5`

---
 include/pybind11/detail/internals.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/pybind11/detail/internals.h b/include/pybind11/detail/internals.h
index 768a290fe2..6be57e7fd5 100644
--- a/include/pybind11/detail/internals.h
+++ b/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)

From 780d3b2ed0ab01012e11d738a369c9f05a35538c Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Mon, 7 Nov 2022 04:55:22 -0800
Subject: [PATCH 07/24] Add a note to docs/classes.rst

---
 docs/classes.rst | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/docs/classes.rst b/docs/classes.rst
index c0c53135b8..ca557936ed 100644
--- a/docs/classes.rst
+++ b/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

From 3373b479b500ce39548da93103dbf5fe1f2c00ec Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Mon, 7 Nov 2022 10:14:27 -0800
Subject: [PATCH 08/24] For compatibility with Google-internal testing,
 test_unnamed_namespace_a & test_unnamed_namespace_b need to work when
 imported in any order.

---
 tests/test_unnamed_namespace_a.cpp | 12 +++++++++++-
 tests/test_unnamed_namespace_a.py  | 28 ++++++++++++++++++++++++++--
 tests/test_unnamed_namespace_b.py  |  5 +----
 3 files changed, 38 insertions(+), 7 deletions(-)

diff --git a/tests/test_unnamed_namespace_a.cpp b/tests/test_unnamed_namespace_a.cpp
index 4f6472c386..7f7eb24aa1 100644
--- a/tests/test_unnamed_namespace_a.cpp
+++ b/tests/test_unnamed_namespace_a.cpp
@@ -5,5 +5,15 @@ struct any_struct {};
 } // namespace
 
 TEST_SUBMODULE(unnamed_namespace_a, m) {
-    py::class_<any_struct>(m, "unnamed_namespace_a_any_struct");
+    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("defined___clang__") =
+#if defined(__clang__)
+        true;
+#else
+        false;
+#endif
 }
diff --git a/tests/test_unnamed_namespace_a.py b/tests/test_unnamed_namespace_a.py
index 995c852e5f..48ae562286 100644
--- a/tests/test_unnamed_namespace_a.py
+++ b/tests/test_unnamed_namespace_a.py
@@ -1,8 +1,32 @@
-# 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_a as m
+from pybind11_tests import unnamed_namespace_b as mb
 
 
+@pytest.mark.xfail(
+    "m.defined___clang__",
+    reason="Known issue with all clang versions: https://github.com/pybind/pybind11/pull/4316",
+    strict=False,
+)
 def test_have_class_any_struct():
     assert m.unnamed_namespace_a_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(
+    "m.defined___clang__",
+    reason="Known issue with all clang versions: https://github.com/pybind/pybind11/pull/4316",
+    strict=True,
+)
+def test_have_both_class_any_struct():
+    assert (
+        m.unnamed_namespace_a_any_struct is not None
+        and mb.unnamed_namespace_b_any_struct is not None
+    )
diff --git a/tests/test_unnamed_namespace_b.py b/tests/test_unnamed_namespace_b.py
index cb4c6f412a..c6645e9c9b 100644
--- a/tests/test_unnamed_namespace_b.py
+++ b/tests/test_unnamed_namespace_b.py
@@ -1,6 +1,3 @@
-# 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
@@ -9,7 +6,7 @@
 @pytest.mark.xfail(
     "m.defined___clang__",
     reason="Known issue with all clang versions: https://github.com/pybind/pybind11/pull/4316",
-    strict=True,
+    strict=False,
 )
 def test_have_class_any_struct():
     assert m.unnamed_namespace_b_any_struct is not None

From 8f0c28e86c3776ed35050c5f4aeafa0789e5e3a1 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Mon, 7 Nov 2022 12:15:25 -0800
Subject: [PATCH 09/24] Trying "__GLIBCXX__ or Windows", based on observations
 from Google-internal testing.

---
 include/pybind11/detail/internals.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/pybind11/detail/internals.h b/include/pybind11/detail/internals.h
index 6be57e7fd5..f0b5518c23 100644
--- a/include/pybind11/detail/internals.h
+++ b/include/pybind11/detail/internals.h
@@ -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(__apple_build_version__)
+#if defined(__GLIBCXX__) || defined(WIN32) || defined(_WIN32)
 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>;

From 883cadfd9b5ec752f39d5cafd422ba62b4dbcecc Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Mon, 7 Nov 2022 14:04:44 -0800
Subject: [PATCH 10/24] Try _LIBCPP_VERSION

---
 include/pybind11/detail/internals.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/pybind11/detail/internals.h b/include/pybind11/detail/internals.h
index f0b5518c23..31e9287318 100644
--- a/include/pybind11/detail/internals.h
+++ b/include/pybind11/detail/internals.h
@@ -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__) || defined(WIN32) || defined(_WIN32)
+#if !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>;

From 72a88958504c94da406bd8c55e1b24db94883282 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Mon, 7 Nov 2022 14:30:06 -0800
Subject: [PATCH 11/24] Account for libc++ behavior in tests and documentation.

---
 docs/classes.rst                   | 4 ++--
 tests/test_unnamed_namespace_a.cpp | 6 ++++++
 tests/test_unnamed_namespace_a.py  | 8 ++++----
 tests/test_unnamed_namespace_b.cpp | 6 ++++++
 tests/test_unnamed_namespace_b.py  | 4 ++--
 5 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/docs/classes.rst b/docs/classes.rst
index ca557936ed..5fbe08870e 100644
--- a/docs/classes.rst
+++ b/docs/classes.rst
@@ -61,8 +61,8 @@ interactive Python session demonstrating this example is shown below:
 .. 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.
+    works reliably only with GCC and MSVC, but not with CLANG, or if libc++ is used.
+    See `#4319 <https://github.com/pybind/pybind11/pull/4319>`_ 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.
diff --git a/tests/test_unnamed_namespace_a.cpp b/tests/test_unnamed_namespace_a.cpp
index 7f7eb24aa1..e538484aa4 100644
--- a/tests/test_unnamed_namespace_a.cpp
+++ b/tests/test_unnamed_namespace_a.cpp
@@ -15,5 +15,11 @@ TEST_SUBMODULE(unnamed_namespace_a, m) {
         true;
 #else
         false;
+#endif
+    m.attr("defined__LIBCPP_VERSION") =
+#if defined(_LIBCPP_VERSION)
+        true;
+#else
+        false;
 #endif
 }
diff --git a/tests/test_unnamed_namespace_a.py b/tests/test_unnamed_namespace_a.py
index 48ae562286..1d3190ddfc 100644
--- a/tests/test_unnamed_namespace_a.py
+++ b/tests/test_unnamed_namespace_a.py
@@ -5,8 +5,8 @@
 
 
 @pytest.mark.xfail(
-    "m.defined___clang__",
-    reason="Known issue with all clang versions: https://github.com/pybind/pybind11/pull/4316",
+    "m.defined___clang__ or m.defined__LIBCPP_VERSION",
+    reason="Known issues: https://github.com/pybind/pybind11/pull/4319",
     strict=False,
 )
 def test_have_class_any_struct():
@@ -21,8 +21,8 @@ def test_have_at_least_one_class_any_struct():
 
 
 @pytest.mark.xfail(
-    "m.defined___clang__",
-    reason="Known issue with all clang versions: https://github.com/pybind/pybind11/pull/4316",
+    "m.defined___clang__ or m.defined__LIBCPP_VERSION",
+    reason="Known issues: https://github.com/pybind/pybind11/pull/4319",
     strict=True,
 )
 def test_have_both_class_any_struct():
diff --git a/tests/test_unnamed_namespace_b.cpp b/tests/test_unnamed_namespace_b.cpp
index 87e5140950..84e3c29447 100644
--- a/tests/test_unnamed_namespace_b.cpp
+++ b/tests/test_unnamed_namespace_b.cpp
@@ -15,5 +15,11 @@ TEST_SUBMODULE(unnamed_namespace_b, m) {
         true;
 #else
         false;
+#endif
+    m.attr("defined__LIBCPP_VERSION") =
+#if defined(_LIBCPP_VERSION)
+        true;
+#else
+        false;
 #endif
 }
diff --git a/tests/test_unnamed_namespace_b.py b/tests/test_unnamed_namespace_b.py
index c6645e9c9b..1e1efac62f 100644
--- a/tests/test_unnamed_namespace_b.py
+++ b/tests/test_unnamed_namespace_b.py
@@ -4,8 +4,8 @@
 
 
 @pytest.mark.xfail(
-    "m.defined___clang__",
-    reason="Known issue with all clang versions: https://github.com/pybind/pybind11/pull/4316",
+    "m.defined___clang__ or m.defined__LIBCPP_VERSION",
+    reason="Known issues: https://github.com/pybind/pybind11/pull/4319",
     strict=False,
 )
 def test_have_class_any_struct():

From 6f1ddb246f803f85fd26203656c811930c1ae97e Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Wed, 9 Nov 2022 20:50:49 -0800
Subject: [PATCH 12/24] Adjust expectations for Windows Clang (and make code
 less redundant).

---
 tests/test_unnamed_namespace_a.cpp |  6 ++++++
 tests/test_unnamed_namespace_a.py  | 20 +++++++++-----------
 tests/test_unnamed_namespace_b.cpp | 12 ------------
 tests/test_unnamed_namespace_b.py  | 11 ++---------
 4 files changed, 17 insertions(+), 32 deletions(-)

diff --git a/tests/test_unnamed_namespace_a.cpp b/tests/test_unnamed_namespace_a.cpp
index e538484aa4..e8704edca0 100644
--- a/tests/test_unnamed_namespace_a.cpp
+++ b/tests/test_unnamed_namespace_a.cpp
@@ -10,6 +10,12 @@ TEST_SUBMODULE(unnamed_namespace_a, m) {
     } else {
         m.attr("unnamed_namespace_a_any_struct") = py::none();
     }
+    m.attr("defined_WIN32_or__WIN32") =
+#if defined(WIN32) || defined(_WIN32)
+        true;
+#else
+        false;
+#endif
     m.attr("defined___clang__") =
 #if defined(__clang__)
         true;
diff --git a/tests/test_unnamed_namespace_a.py b/tests/test_unnamed_namespace_a.py
index 1d3190ddfc..be4f4bfe9b 100644
--- a/tests/test_unnamed_namespace_a.py
+++ b/tests/test_unnamed_namespace_a.py
@@ -3,14 +3,16 @@
 from pybind11_tests import unnamed_namespace_a as m
 from pybind11_tests import unnamed_namespace_b as mb
 
+XFAIL_CONDITION = "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(
-    "m.defined___clang__ or m.defined__LIBCPP_VERSION",
-    reason="Known issues: https://github.com/pybind/pybind11/pull/4319",
-    strict=False,
+
+@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():
-    assert m.unnamed_namespace_a_any_struct is not None
+def test_have_class_any_struct(any_struct):
+    assert any_struct is not None
 
 
 def test_have_at_least_one_class_any_struct():
@@ -20,11 +22,7 @@ def test_have_at_least_one_class_any_struct():
     )
 
 
-@pytest.mark.xfail(
-    "m.defined___clang__ or m.defined__LIBCPP_VERSION",
-    reason="Known issues: https://github.com/pybind/pybind11/pull/4319",
-    strict=True,
-)
+@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
diff --git a/tests/test_unnamed_namespace_b.cpp b/tests/test_unnamed_namespace_b.cpp
index 84e3c29447..f97757a7de 100644
--- a/tests/test_unnamed_namespace_b.cpp
+++ b/tests/test_unnamed_namespace_b.cpp
@@ -10,16 +10,4 @@ TEST_SUBMODULE(unnamed_namespace_b, m) {
     } else {
         m.attr("unnamed_namespace_b_any_struct") = py::none();
     }
-    m.attr("defined___clang__") =
-#if defined(__clang__)
-        true;
-#else
-        false;
-#endif
-    m.attr("defined__LIBCPP_VERSION") =
-#if defined(_LIBCPP_VERSION)
-        true;
-#else
-        false;
-#endif
 }
diff --git a/tests/test_unnamed_namespace_b.py b/tests/test_unnamed_namespace_b.py
index 1e1efac62f..4bcaa7a6c5 100644
--- a/tests/test_unnamed_namespace_b.py
+++ b/tests/test_unnamed_namespace_b.py
@@ -1,12 +1,5 @@
-import pytest
-
 from pybind11_tests import unnamed_namespace_b as m
 
 
-@pytest.mark.xfail(
-    "m.defined___clang__ or m.defined__LIBCPP_VERSION",
-    reason="Known issues: https://github.com/pybind/pybind11/pull/4319",
-    strict=False,
-)
-def test_have_class_any_struct():
-    assert m.unnamed_namespace_b_any_struct is not None
+def test_have_attr_any_struct():
+    assert hasattr(m, "unnamed_namespace_b_any_struct")

From a1175edf90bff26449a27a6ad67b2c3f9ee0463e Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Wed, 9 Nov 2022 21:15:07 -0800
Subject: [PATCH 13/24] Add WindowsClang to ci.yml

Added block transferred from PR #4321
---
 .github/workflows/ci.yml | 65 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a11cae1ab0..1c24f4f549 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -967,3 +967,68 @@ jobs:
 
     - name: Interface test C++17
       run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build3 --target test_cmake_build
+
+  WindowsClang:
+
+    strategy:
+      matrix:
+        os: [windows-latest]
+        python: ['3.10']
+
+    runs-on: "${{ matrix.os }}"
+
+    name: "🐍 ${{ matrix.python }} • ${{ matrix.os }}"
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Set up Clang
+        uses: egor-tensin/setup-clang@v1
+
+      - name: Setup Python ${{ matrix.python }}
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python }}
+
+      - name: Update CMake
+        uses: jwlawson/actions-setup-cmake@v1.13
+
+      - name: Install ninja-build tool
+        uses: seanmiddleditch/gha-setup-ninja@v3
+
+      - name: Prepare env
+        run: python -m pip install -r tests/requirements.txt
+
+      - name: Show Clang++ version
+        run: clang++ --version
+
+      - name: Show CMake version
+        run: cmake --version
+
+      # TODO: WERROR=ON
+      - name: Configure Clang
+        run: >
+          cmake -G Ninja -S . -B .
+          -DCMAKE_VERBOSE_MAKEFILE=ON
+          -DPYBIND11_WERROR=OFF
+          -DPYBIND11_SIMPLE_GIL_MANAGEMENT=OFF
+          -DDOWNLOAD_CATCH=ON
+          -DDOWNLOAD_EIGEN=ON
+          -DCMAKE_CXX_COMPILER=clang++
+          -DCMAKE_CXX_STANDARD=17
+
+      - name: Build Clang
+        run: cmake --build . -j 2
+
+      - name: Python tests Clang
+        run: cmake --build . --target pytest -j 2
+
+      - name: C++ tests
+        run: cmake --build . --target cpptest -j 2
+
+      - name: Interface test Clang
+        run: cmake --build . --target test_cmake_build -j 2
+
+      - name: Clean directory
+        run: git clean -fdx

From 5631d94e4ad33ac98bebe42255ec599fd9456344 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Wed, 9 Nov 2022 23:23:27 -0800
Subject: [PATCH 14/24] Add clang-latest to name that appears in the GitHub
 Actions web view.

---
 .github/workflows/ci.yml | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 1c24f4f549..d0c4c850f8 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -968,7 +968,7 @@ jobs:
     - name: Interface test C++17
       run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build3 --target test_cmake_build
 
-  WindowsClang:
+  windows_clang:
 
     strategy:
       matrix:
@@ -977,7 +977,7 @@ jobs:
 
     runs-on: "${{ matrix.os }}"
 
-    name: "🐍 ${{ matrix.python }} • ${{ matrix.os }}"
+    name: "🐍 ${{ matrix.python }} • ${{ matrix.os }} • clang-latest"
 
     steps:
       - name: Checkout

From 81a4e7ba8c562a10baeba84ec7a0651a0a6d2d7b Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Thu, 10 Nov 2022 07:04:28 -0800
Subject: [PATCH 15/24] Tweak the note in classes.rst again.

---
 docs/classes.rst | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/docs/classes.rst b/docs/classes.rst
index 5fbe08870e..52cd52da3f 100644
--- a/docs/classes.rst
+++ b/docs/classes.rst
@@ -61,8 +61,9 @@ interactive Python session demonstrating this example is shown below:
 .. note::
 
     Binding C++ types in unnamed namespaces (also known as anonymous namespaces)
-    works reliably only with GCC and MSVC, but not with CLANG, or if libc++ is used.
-    See `#4319 <https://github.com/pybind/pybind11/pull/4319>`_ for background.
+    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.

From 7075e00b7dc29ac2b789991542a0e2e4cdaea4e8 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Thu, 10 Nov 2022 07:26:46 -0800
Subject: [PATCH 16/24] Add `pip install --upgrade pip`, Show env, cosmetic
 changes

Already tested under PR #4321
---
 .github/workflows/ci.yml | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d0c4c850f8..adbbf626f1 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -980,6 +980,9 @@ jobs:
     name: "🐍 ${{ matrix.python }} • ${{ matrix.os }} • clang-latest"
 
     steps:
+      - name: Show env
+        run: env
+
       - name: Checkout
         uses: actions/checkout@v3
 
@@ -997,8 +1000,10 @@ jobs:
       - name: Install ninja-build tool
         uses: seanmiddleditch/gha-setup-ninja@v3
 
-      - name: Prepare env
-        run: python -m pip install -r tests/requirements.txt
+      - name: Run pip installs
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install -r tests/requirements.txt
 
       - name: Show Clang++ version
         run: clang++ --version
@@ -1018,16 +1023,16 @@ jobs:
           -DCMAKE_CXX_COMPILER=clang++
           -DCMAKE_CXX_STANDARD=17
 
-      - name: Build Clang
+      - name: Build
         run: cmake --build . -j 2
 
-      - name: Python tests Clang
+      - name: Python tests
         run: cmake --build . --target pytest -j 2
 
       - name: C++ tests
         run: cmake --build . --target cpptest -j 2
 
-      - name: Interface test Clang
+      - name: Interface test
         run: cmake --build . --target test_cmake_build -j 2
 
       - name: Clean directory

From b84a1630dace2ba82c865fdbbcc75d84a8c32327 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Thu, 10 Nov 2022 07:29:07 -0800
Subject: [PATCH 17/24] Add macos_brew_install_llvm to ci.yml

Added block transferred from PR #4324
---
 .github/workflows/ci.yml | 65 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index adbbf626f1..c16b07423e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1037,3 +1037,68 @@ jobs:
 
       - name: Clean directory
         run: git clean -fdx
+
+  macos_brew_install_llvm:
+    name: "macos-latest • brew install llvm"
+    runs-on: macos-latest
+
+    env:
+      # https://apple.stackexchange.com/questions/227026/how-to-install-recent-clang-with-homebrew
+      LDFLAGS: '-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib'
+
+    steps:
+      - name: Update PATH
+        run: echo "/usr/local/opt/llvm/bin" >> $GITHUB_PATH
+
+      - name: Show env
+        run: env
+
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Show Clang++ version before brew install llvm
+        run: clang++ --version
+
+      - name: brew install llvm
+        run: brew install llvm
+
+      - name: Show Clang++ version after brew install llvm
+        run: clang++ --version
+
+      - name: Update CMake
+        uses: jwlawson/actions-setup-cmake@v1.13
+
+      - name: Run pip installs
+        run: |
+          python3 -m pip install --upgrade pip
+          python3 -m pip install -r tests/requirements.txt
+
+      - name: Show CMake version
+        run: cmake --version
+
+      - name: CMake Configure
+        run: >
+          cmake -S . -B .
+          -DCMAKE_VERBOSE_MAKEFILE=ON
+          -DPYBIND11_WERROR=ON
+          -DPYBIND11_SIMPLE_GIL_MANAGEMENT=OFF
+          -DDOWNLOAD_CATCH=ON
+          -DDOWNLOAD_EIGEN=ON
+          -DCMAKE_CXX_COMPILER=clang++
+          -DCMAKE_CXX_STANDARD=17
+          -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+
+      - name: Build
+        run: cmake --build . -j 2
+
+      - name: Python tests
+        run: cmake --build . --target pytest -j 2
+
+      - name: C++ tests
+        run: cmake --build . --target cpptest -j 2
+
+      - name: Interface test
+        run: cmake --build . --target test_cmake_build -j 2
+
+      - name: Clean directory
+        run: git clean -fdx

From 4f06c061b6ea6bd8ffcb45c2cdf02c50f86017f4 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Thu, 10 Nov 2022 06:29:12 -0800
Subject: [PATCH 18/24] `test_cross_module_exception_translator` xfail
 'Homebrew Clang'

---
 tests/test_exceptions.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/tests/test_exceptions.py b/tests/test_exceptions.py
index 7eb1a9d62c..0d2c808143 100644
--- a/tests/test_exceptions.py
+++ b/tests/test_exceptions.py
@@ -4,6 +4,7 @@
 
 import env
 import pybind11_cross_module_tests as cm
+import pybind11_tests  # noqa: F401
 from pybind11_tests import exceptions as m
 
 
@@ -72,9 +73,9 @@ def test_cross_module_exceptions(msg):
 
 # TODO: FIXME
 @pytest.mark.xfail(
-    "env.PYPY and env.MACOS",
+    "env.MACOS and (env.PYPY or pybind11_tests.compiler_info.startswith('Homebrew Clang'))",
     raises=RuntimeError,
-    reason="Expected failure with PyPy and libc++ (Issue #2847 & PR #2999)",
+    reason="See Issue #2847, PR #2999, PR #4324",
 )
 def test_cross_module_exception_translator():
     with pytest.raises(KeyError):

From 2180d04f9a6d91f75fb74d45709c154a75961591 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Mon, 20 Mar 2023 13:46:19 -0700
Subject: [PATCH 19/24] Revert back to base version of .github/workflows/ci.yml
 (the ci.yml changes were merged under #4323 and #4326)

---
 .github/workflows/ci.yml | 135 ---------------------------------------
 1 file changed, 135 deletions(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c16b07423e..a11cae1ab0 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -967,138 +967,3 @@ jobs:
 
     - name: Interface test C++17
       run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build3 --target test_cmake_build
-
-  windows_clang:
-
-    strategy:
-      matrix:
-        os: [windows-latest]
-        python: ['3.10']
-
-    runs-on: "${{ matrix.os }}"
-
-    name: "🐍 ${{ matrix.python }} • ${{ matrix.os }} • clang-latest"
-
-    steps:
-      - name: Show env
-        run: env
-
-      - name: Checkout
-        uses: actions/checkout@v3
-
-      - name: Set up Clang
-        uses: egor-tensin/setup-clang@v1
-
-      - name: Setup Python ${{ matrix.python }}
-        uses: actions/setup-python@v4
-        with:
-          python-version: ${{ matrix.python }}
-
-      - name: Update CMake
-        uses: jwlawson/actions-setup-cmake@v1.13
-
-      - name: Install ninja-build tool
-        uses: seanmiddleditch/gha-setup-ninja@v3
-
-      - name: Run pip installs
-        run: |
-          python -m pip install --upgrade pip
-          python -m pip install -r tests/requirements.txt
-
-      - name: Show Clang++ version
-        run: clang++ --version
-
-      - name: Show CMake version
-        run: cmake --version
-
-      # TODO: WERROR=ON
-      - name: Configure Clang
-        run: >
-          cmake -G Ninja -S . -B .
-          -DCMAKE_VERBOSE_MAKEFILE=ON
-          -DPYBIND11_WERROR=OFF
-          -DPYBIND11_SIMPLE_GIL_MANAGEMENT=OFF
-          -DDOWNLOAD_CATCH=ON
-          -DDOWNLOAD_EIGEN=ON
-          -DCMAKE_CXX_COMPILER=clang++
-          -DCMAKE_CXX_STANDARD=17
-
-      - name: Build
-        run: cmake --build . -j 2
-
-      - name: Python tests
-        run: cmake --build . --target pytest -j 2
-
-      - name: C++ tests
-        run: cmake --build . --target cpptest -j 2
-
-      - name: Interface test
-        run: cmake --build . --target test_cmake_build -j 2
-
-      - name: Clean directory
-        run: git clean -fdx
-
-  macos_brew_install_llvm:
-    name: "macos-latest • brew install llvm"
-    runs-on: macos-latest
-
-    env:
-      # https://apple.stackexchange.com/questions/227026/how-to-install-recent-clang-with-homebrew
-      LDFLAGS: '-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib'
-
-    steps:
-      - name: Update PATH
-        run: echo "/usr/local/opt/llvm/bin" >> $GITHUB_PATH
-
-      - name: Show env
-        run: env
-
-      - name: Checkout
-        uses: actions/checkout@v3
-
-      - name: Show Clang++ version before brew install llvm
-        run: clang++ --version
-
-      - name: brew install llvm
-        run: brew install llvm
-
-      - name: Show Clang++ version after brew install llvm
-        run: clang++ --version
-
-      - name: Update CMake
-        uses: jwlawson/actions-setup-cmake@v1.13
-
-      - name: Run pip installs
-        run: |
-          python3 -m pip install --upgrade pip
-          python3 -m pip install -r tests/requirements.txt
-
-      - name: Show CMake version
-        run: cmake --version
-
-      - name: CMake Configure
-        run: >
-          cmake -S . -B .
-          -DCMAKE_VERBOSE_MAKEFILE=ON
-          -DPYBIND11_WERROR=ON
-          -DPYBIND11_SIMPLE_GIL_MANAGEMENT=OFF
-          -DDOWNLOAD_CATCH=ON
-          -DDOWNLOAD_EIGEN=ON
-          -DCMAKE_CXX_COMPILER=clang++
-          -DCMAKE_CXX_STANDARD=17
-          -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
-
-      - name: Build
-        run: cmake --build . -j 2
-
-      - name: Python tests
-        run: cmake --build . --target pytest -j 2
-
-      - name: C++ tests
-        run: cmake --build . --target cpptest -j 2
-
-      - name: Interface test
-        run: cmake --build . --target test_cmake_build -j 2
-
-      - name: Clean directory
-        run: git clean -fdx

From b882060643b10096ffabccf7cb9b63178cc3d92d Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Mon, 20 Mar 2023 13:49:44 -0700
Subject: [PATCH 20/24] Fixes for ruff

---
 tests/test_unnamed_namespace_a.py | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/tests/test_unnamed_namespace_a.py b/tests/test_unnamed_namespace_a.py
index be4f4bfe9b..373693cd5c 100644
--- a/tests/test_unnamed_namespace_a.py
+++ b/tests/test_unnamed_namespace_a.py
@@ -9,7 +9,7 @@
 
 @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)
+    "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
@@ -24,7 +24,5 @@ def test_have_at_least_one_class_any_struct():
 
 @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
-        and mb.unnamed_namespace_b_any_struct is not None
-    )
+    assert m.unnamed_namespace_a_any_struct is not None
+    assert mb.unnamed_namespace_b_any_struct is not None

From f1c3055a2decda121302343524db2277aa9ec9d9 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Tue, 28 Mar 2023 16:17:37 -0700
Subject: [PATCH 21/24] Make updated condition in internals.h dependent on ABI
 version.

---
 include/pybind11/detail/internals.h | 3 ++-
 tests/test_unnamed_namespace_a.cpp  | 7 +++++++
 tests/test_unnamed_namespace_a.py   | 8 +++++++-
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/include/pybind11/detail/internals.h b/include/pybind11/detail/internals.h
index e42ea40305..aaa7f8686e 100644
--- a/include/pybind11/detail/internals.h
+++ b/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(_LIBCPP_VERSION)
+#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>;
diff --git a/tests/test_unnamed_namespace_a.cpp b/tests/test_unnamed_namespace_a.cpp
index e8704edca0..2152e64bd7 100644
--- a/tests/test_unnamed_namespace_a.cpp
+++ b/tests/test_unnamed_namespace_a.cpp
@@ -10,6 +10,7 @@ TEST_SUBMODULE(unnamed_namespace_a, m) {
     } 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;
@@ -27,5 +28,11 @@ TEST_SUBMODULE(unnamed_namespace_a, m) {
         true;
 #else
         false;
+#endif
+    m.attr("defined___GLIBCXX__") =
+#if defined(__GLIBCXX__)
+        true;
+#else
+        false;
 #endif
 }
diff --git a/tests/test_unnamed_namespace_a.py b/tests/test_unnamed_namespace_a.py
index 373693cd5c..9d9856c5a4 100644
--- a/tests/test_unnamed_namespace_a.py
+++ b/tests/test_unnamed_namespace_a.py
@@ -3,7 +3,13 @@
 from pybind11_tests import unnamed_namespace_a as m
 from pybind11_tests import unnamed_namespace_b as mb
 
-XFAIL_CONDITION = "not m.defined_WIN32_or__WIN32 and (m.defined___clang__ or m.defined__LIBCPP_VERSION)"
+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"
 
 

From 0ff73a9831b8e7142dfbff626f5a006da2efcf79 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Thu, 30 Mar 2023 08:59:13 -0700
Subject: [PATCH 22/24] Remove PYBIND11_TEST_OVERRIDE when testing with
 PYBIND11_INTERNALS_VERSION=10000000

---
 .github/workflows/ci.yml       | 1 -
 .github/workflows/upstream.yml | 1 -
 2 files changed, 2 deletions(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 8befccdbc2..f63abbc2d4 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -164,7 +164,6 @@ jobs:
         -DDOWNLOAD_EIGEN=ON
         -DCMAKE_CXX_STANDARD=17
         -DPYBIND11_INTERNALS_VERSION=10000000
-        "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
         ${{ matrix.args }}
 
     - name: Build (unstable ABI)
diff --git a/.github/workflows/upstream.yml b/.github/workflows/upstream.yml
index b7e14f72e8..10b017c518 100644
--- a/.github/workflows/upstream.yml
+++ b/.github/workflows/upstream.yml
@@ -95,7 +95,6 @@ jobs:
         -DDOWNLOAD_EIGEN=ON
         -DCMAKE_CXX_STANDARD=17
         -DPYBIND11_INTERNALS_VERSION=10000000
-        "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
 
     - name: Build (unstable ABI)
       run: cmake --build build17max -j 2

From d0276c01cd9e0a24aafe74c698d666908a31a397 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Thu, 20 Apr 2023 23:14:38 -0700
Subject: [PATCH 23/24] Selectively exercise cmake `-DPYBIND11_TEST_OVERRIDE`:
 ubuntu, macos, windows

Extra work added to quick jobs, based on timings below, to not increase the GHA start-to-last-job-finished time.

```
Duration
^              Number of pytest runs
^              ^ Job identifier
^              ^ ^
0:03:48.024227 1 1___3___Clang_3.6___C++11___x64.txt
0:03:58.992814 1 2___3___Clang_3.7___C++11___x64.txt
0:04:25.758942 1 1___3.7___Debian___x86____Install.txt
0:04:50.148276 1 4___3___Clang_7___C++11___x64.txt
0:04:55.784558 1 13___3___Clang_15___C++20___x64.txt
0:04:57.048754 1 6___3___Clang_dev___C++11___x64.txt
0:05:00.485181 1 7___3___Clang_5___C++14___x64.txt
0:05:03.744964 1 2___3___almalinux8___x64.txt
0:05:06.222752 1 5___3___Clang_9___C++11___x64.txt
0:05:11.767022 1 2___3___GCC_7___C++17__x64.txt
0:05:18.634930 1 2___3.11__deadsnakes____x64.txt
0:05:22.810995 1 1___3___GCC_7___C++11__x64.txt
0:05:25.275317 1 12___3___Clang_14___C++20___x64.txt
0:05:32.058174 1 5___3___GCC_10___C++17__x64.txt
0:05:39.381351 1 7___3___GCC_12___C++20__x64.txt
0:05:40.502252 1 8___3___Clang_10___C++17___x64.txt
0:05:59.344905 1 3___3___Clang_3.9___C++11___x64.txt
0:06:10.825147 1 6___3___GCC_11___C++20__x64.txt
0:06:20.655443 1 3___3___almalinux9___x64.txt
0:06:22.472061 1 3___3___GCC_8___C++14__x64.txt
0:06:42.647406 1 11___3___Clang_13___C++20___x64.txt
0:06:53.352720 1 1___3.10___CUDA_11.7___Ubuntu_22.04.txt
0:07:07.357801 1 2___3.7___MSVC_2019___x86_-DCMAKE_CXX_STANDARD=14.txt
0:07:09.057603 1 1___3___centos7___x64.txt
0:07:15.546282 1 1___3.8___MSVC_2019__Debug____x86_-DCMAKE_CXX_STANDARD=17.txt
0:07:22.566022 1 4___3___GCC_8___C++17__x64.txt
0:08:13.592674 1 2___3.9___MSVC_2019__Debug____x86_-DCMAKE_CXX_STANDARD=20.txt
0:08:16.422768 1 9___3___Clang_11___C++20___x64.txt
0:08:21.168457 1 3___3.8___MSVC_2019___x86_-DCMAKE_CXX_STANDARD=17.txt
0:08:27.129468 1 10___3___Clang_12___C++20___x64.txt
0:09:35.045470 1 1___3.10___windows-latest___clang-latest.txt
0:09:57.361843 1 1___3.9___MSVC_2022_C++20___x64.txt
0:10:35.187767 1 1___3.6___MSVC_2019___x86.txt
0:11:14.691200 4 2___3.9___ubuntu-20.04___x64.txt
0:11:37.701167 1 1_macos-latest___brew_install_llvm.txt
0:11:38.688299 4 4___3.11___ubuntu-20.04___x64.txt
0:11:52.720216 1 4___3.9___MSVC_2019___x86_-DCMAKE_CXX_STANDARD=20.txt
0:13:23.456591 4 6___pypy-3.8___ubuntu-20.04___x64_-DPYBIND11_FINDPYTHON=ON.txt
0:13:25.863592 2 1___3___ICC_latest___x64.txt
0:13:32.411758 3 9___3.9___windows-2022___x64.txt
0:13:45.473377 4 3___3.10___ubuntu-20.04___x64.txt
0:13:55.366447 4 5___pypy-3.7___ubuntu-20.04___x64.txt
0:13:57.969502 3 10___3.10___windows-2022___x64.txt
0:14:19.837475 3 11___3.11___windows-2022___x64.txt
0:14:33.316770 4 1___3.6___ubuntu-20.04___x64_-DPYBIND11_FINDPYTHON=ON_-DCMA.txt
0:15:34.449278 4 22___3.6___windows-2019___x64_-DPYBIND11_FINDPYTHON=ON.txt
0:16:25.189055 2 1___3.9-dbg__deadsnakes____Valgrind___x64.txt
0:17:20.956667 4 15___3.6___macos-latest___x64.txt
0:17:27.513891 4 23___3.9___windows-2019___x64.txt
0:17:58.783286 3 8___3.6___windows-2022___x64.txt
0:18:25.917828 4 7___pypy-3.9___ubuntu-20.04___x64.txt
0:19:17.399820 3 13___pypy-3.8___windows-2022___x64.txt
0:19:45.002122 3 12___pypy-3.7___windows-2022___x64.txt
0:20:03.201926 4 16___3.9___macos-latest___x64.txt
0:20:15.415178 4 17___3.10___macos-latest___x64.txt
0:20:20.263216 4 20___pypy-3.8___macos-latest___x64.txt
0:20:31.998226 3 1___3___windows-latest___mingw64.txt
0:20:40.812286 4 18___3.11___macos-latest___x64.txt
0:22:47.714749 4 19___pypy-3.7___macos-latest___x64.txt
0:23:04.435859 3 2___3___windows-latest___mingw32.txt
0:25:48.719597 3 14___pypy-3.9___windows-2022___x64.txt
0:26:01.211688 4 21___pypy-3.9___macos-latest___x64.txt
0:28:19.971015 1 1___3___CentOS7__PGI_22.9___x64.txt
```
---
 .github/workflows/ci.yml | 51 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index f63abbc2d4..a2c2c310be 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -496,6 +496,24 @@ jobs:
     - name: Interface test
       run: cmake --build build --target test_cmake_build
 
+    - name: Configure - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+      if: matrix.gcc == '12'
+      shell: bash
+      run: >
+        cmake -S . -B build_partial
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DCMAKE_CXX_STANDARD=${{ matrix.std }}
+        -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+        "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
+
+    - name: Build - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+      if: matrix.gcc == '12'
+      run: cmake --build build_partial -j 2
+
+    - name: Python tests - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+      if: matrix.gcc == '12'
+      run: cmake --build build_partial --target pytest
 
   # Testing on ICC using the oneAPI apt repo
   icc:
@@ -888,6 +906,21 @@ jobs:
     - name: Interface test C++20
       run: cmake --build build --target test_cmake_build
 
+    - name: Configure C++20 - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+      run: >
+        cmake -S . -B build_partial
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DDOWNLOAD_EIGEN=ON
+        -DCMAKE_CXX_STANDARD=20
+        "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
+
+    - name: Build C++20 - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+      run: cmake --build build_partial -j 2
+
+    - name: Python tests - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+      run: cmake --build build_partial --target pytest
+
   mingw:
     name: "🐍 3 • windows-latest • ${{ matrix.sys }}"
     runs-on: windows-latest
@@ -1104,5 +1137,23 @@ jobs:
       - name: Interface test
         run: cmake --build . --target test_cmake_build -j 2
 
+      - name: CMake Configure - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+        run: >
+          cmake -S . -B build_partial
+          -DPYBIND11_WERROR=ON
+          -DPYBIND11_SIMPLE_GIL_MANAGEMENT=OFF
+          -DDOWNLOAD_CATCH=ON
+          -DDOWNLOAD_EIGEN=ON
+          -DCMAKE_CXX_COMPILER=clang++
+          -DCMAKE_CXX_STANDARD=17
+          -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+          "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
+
+      - name: Build - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+        run: cmake --build build_partial -j 2
+
+      - name: Python tests - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+        run: cmake --build build_partial --target pytest -j 2
+
       - name: Clean directory
         run: git clean -fdx

From 1b4a508d5e5d44d74664d8fa06c27cea568ff36a Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Tue, 25 Apr 2023 10:35:07 -0700
Subject: [PATCH 24/24] Update skipif for Python 3.12a7 (the WIP needs to be
 handled in a separate PR).

---
 tests/test_exceptions.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/test_exceptions.py b/tests/test_exceptions.py
index 1b4c89add1..a92ab59a34 100644
--- a/tests/test_exceptions.py
+++ b/tests/test_exceptions.py
@@ -319,7 +319,7 @@ def test_error_already_set_what_with_happy_exceptions(
 
 @pytest.mark.skipif(
     # Intentionally very specific:
-    "sys.version_info == (3, 12, 0, 'alpha', 6)",
+    "sys.version_info == (3, 12, 0, 'alpha', 7)",
     reason="WIP: https://github.com/python/cpython/issues/102594",
 )
 @pytest.mark.skipif("env.PYPY", reason="PyErr_NormalizeException Segmentation fault")