Follow-on to #30093 (correction) (#30098)

* Bring back `std::addressof(src)` incorrectly removed with commit bcc40b2

Silly oversight: the `operator&() const` overload was missing in the test code.

* Change comment as suggested by @rainwoodman.

include/pybind11/detail/smart_holder_type_casters.h

@@ -757,7 +757,7 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
             || policy == return_value_policy::_clif_automatic) {
             policy = return_value_policy::copy;
         }
-        return cast(&src, policy, parent);
+        return cast(std::addressof(src), policy, parent);
         // type_caster_base END
     }
 

include/pybind11/detail/type_caster_base.h

@@ -1116,7 +1116,7 @@ class type_caster_base : public type_caster_generic {
             || policy == return_value_policy::automatic_reference) {
             policy = return_value_policy::copy;
         }
-        return cast(&src, policy, parent);
+        return cast(std::addressof(src), policy, parent);
     }
 
     static handle cast(itype &&src, return_value_policy, handle parent) {

tests/pybind11_tests.h

@@ -57,7 +57,9 @@ union IntFloat {
 class UnusualOpRef {
 public:
     using NonTrivialType = std::shared_ptr<int>; // Almost any non-trivial type will do.
-    NonTrivialType operator&() { return non_trivial_member; } // UNUSUAL operator.
+    // Overriding operator& should not break pybind11.
+    NonTrivialType operator&() { return non_trivial_member; }
+    const NonTrivialType operator&() const { return non_trivial_member; }
 
 private:
     NonTrivialType non_trivial_member;