fix: avoid copy constructor instantiation in shared_ptr fallback cast (#6028)

* tests: add regressions for shared_ptr reference_internal fallback

* fix: avoid copy constructor instantiation in shared_ptr fallback cast

* Remove stray empty line

* tests: rename PyTorch shared_ptr regression test files

* refactor: add cast_non_owning helper for reference-like casts

Name the non-owning generic cast path so callers do not have to rediscover that
reference-like policies must pass null copy/move constructor callbacks. This
keeps the shared_ptr reference_internal fallback self-documenting and points
future maintainers toward the safe API.

Made-with: Cursor

* tests: guard deprecated-copy warning probes with __has_warning

Use __has_warning for the Clang-only regression test so older compiler jobs skip
unsupported warning groups instead of failing with -Wunknown-warning-option. A
simple __clang_major__ >= 13 guard would be shorter, but it bakes in a version
cutoff; __has_warning is slightly more verbose while being more robust to
vendor builds, backports, and future packaging differences.

Made-with: Cursor

---------

Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>

include/pybind11/cast.h

@@ -1026,7 +1026,7 @@ public:
         }
 
         if (parent) {
-            return type_caster_base<type>::cast(
+            return type_caster_generic::cast_non_owning(
                 srcs, return_value_policy::reference_internal, parent);
         }
 

include/pybind11/detail/type_caster_base.h

@@ -1004,6 +1004,18 @@ public:
         return cast(srcs, policy, parent, copy_constructor, move_constructor, existing_holder);
     }
 
+    static handle cast_non_owning(const cast_sources &srcs,
+                                  return_value_policy policy,
+                                  handle parent,
+                                  const void *existing_holder = nullptr) {
+        // Reference-like policies alias an existing C++ object instead of creating
+        // a new one, so copy/move constructor callbacks must remain null here.
+        assert(policy == return_value_policy::reference
+               || policy == return_value_policy::reference_internal
+               || policy == return_value_policy::automatic_reference);
+        return cast(srcs, policy, parent, nullptr, nullptr, existing_holder);
+    }
+
     PYBIND11_NOINLINE static handle cast(const cast_sources &srcs,
                                          return_value_policy policy,
                                          handle parent,

tests/CMakeLists.txt

@@ -167,6 +167,7 @@ set(PYBIND11_TEST_FILES
     test_operator_overloading
     test_pickling
     test_potentially_slicing_weak_ptr
+    test_pytorch_shared_ptr_cast_regression
     test_python_multiple_inheritance
     test_pytypes
     test_scoped_critical_section

tests/test_class_sh_property.py

@@ -204,3 +204,15 @@ def test_non_smart_holder_member_type_with_smart_holder_owner_aliases_member():
     legacy = obj.legacy
     legacy.value = 13
     assert obj.legacy.value == 13
+
+
+def test_non_smart_holder_member_type_with_smart_holder_owner_aliases_member_multiple_reads():
+    obj = m.ShWithSimpleStructMember()
+
+    a = obj.legacy
+    b = obj.legacy
+
+    a.value = 13
+
+    assert b.value == 13
+    assert obj.legacy.value == 13

tests/test_pytorch_shared_ptr_cast_regression.cpp

@@ -0,0 +1,62 @@
+#include "pybind11_tests.h"
+
+#include <memory>
+#include <string>
+
+#if defined(__clang__)
+#    if __has_warning("-Wdeprecated-copy-with-user-provided-dtor")
+#        pragma clang diagnostic error "-Wdeprecated-copy-with-user-provided-dtor"
+#    endif
+#    if __has_warning("-Wdeprecated-copy-with-dtor")
+#        pragma clang diagnostic error "-Wdeprecated-copy-with-dtor"
+#    endif
+#endif
+
+namespace test_pytorch_regressions {
+
+// Directly extracted from PyTorch patterns that regressed in CI.
+struct TracingState : std::enable_shared_from_this<TracingState> {
+    TracingState() = default;
+    ~TracingState() = default;
+    int value = 0;
+};
+
+const std::shared_ptr<TracingState> &get_tracing_state() {
+    static std::shared_ptr<TracingState> state = std::make_shared<TracingState>();
+    return state;
+}
+
+struct InterfaceType {
+    ~InterfaceType() = default;
+    int value = 0;
+};
+using InterfaceTypePtr = std::shared_ptr<InterfaceType>;
+
+struct CompilationUnit {
+    InterfaceTypePtr iface = std::make_shared<InterfaceType>();
+
+    InterfaceTypePtr get_interface(const std::string &) const { return iface; }
+};
+
+} // namespace test_pytorch_regressions
+
+TEST_SUBMODULE(pybind11_pytorch_regressions, m) {
+    using namespace test_pytorch_regressions;
+
+    py::class_<TracingState, std::shared_ptr<TracingState>>(m, "TracingState")
+        .def(py::init<>())
+        .def_readwrite("value", &TracingState::value);
+
+    m.def("_get_tracing_state", []() { return get_tracing_state(); });
+
+    py::class_<InterfaceType, InterfaceTypePtr>(m, "InterfaceType")
+        .def(py::init<>())
+        .def_readwrite("value", &InterfaceType::value);
+
+    py::class_<CompilationUnit, std::shared_ptr<CompilationUnit>>(m, "CompilationUnit")
+        .def(py::init<>())
+        .def("get_interface",
+             [](const std::shared_ptr<CompilationUnit> &self, const std::string &name) {
+                 return self->get_interface(name);
+             });
+}

tests/test_pytorch_shared_ptr_cast_regression.py

@@ -0,0 +1,25 @@
+from __future__ import annotations
+
+from pybind11_tests import pybind11_pytorch_regressions as m
+
+
+def test_pytorch_like_get_tracing_state_aliases_singleton_shared_ptr():
+    a = m._get_tracing_state()
+    b = m._get_tracing_state()
+
+    a.value = 17
+
+    assert b.value == 17
+    assert m._get_tracing_state().value == 17
+
+
+def test_pytorch_like_compilation_unit_get_interface_aliases_member_shared_ptr():
+    cu = m.CompilationUnit()
+
+    a = cu.get_interface("iface")
+    b = cu.get_interface("iface")
+
+    a.value = 23
+
+    assert b.value == 23
+    assert cu.get_interface("iface").value == 23