Minimal reproducer for clash when binding types defined in the unnamed namespace.

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 +1,5 @@
+from pybind11_tests import unnamed_namespace_a as m
+
+
+def test_have_type():
+    assert hasattr(m, "unnamed_namespace_a_any_struct")

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");
+}

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")