smart_holder fixups (#3012)

* Drop constraints on casting of std::shared_ptr

std::shared_ptrs can be shared across python and C++ by design.

* Correctly report casting error

It is important to return an empty handle.
Simply returning None, would skip the error handling in
simple_collector / unpacking_collector, although a python exception is set.
A function call would then be processed with a (wrong) None argument!

* Return None for nullptr

* Revert "Drop constraints on casting of std::shared_ptr"

This reverts commit 7cf53ae.

Author: rhaschke

include/pybind11/detail/smart_holder_type_casters.h

@@ -624,15 +624,18 @@ struct smart_holder_type_caster<std::shared_ptr<T>> : smart_holder_type_caster_l
 
     static handle cast(const std::shared_ptr<T> &src, return_value_policy policy, handle parent) {
         if (policy != return_value_policy::automatic
-            && policy != return_value_policy::reference_internal) {
+            && policy != return_value_policy::reference_internal
+            && policy != return_value_policy::automatic_reference) {
             // SMART_HOLDER_WIP: IMPROVABLE: Error message.
             throw cast_error("Invalid return_value_policy for shared_ptr.");
         }
+        if (!src)
+            return none().release();
 
         auto src_raw_ptr = src.get();
         auto st          = type_caster_base<T>::src_and_type(src_raw_ptr);
-        if (st.first == nullptr)
-            return none().release(); // PyErr was set already.
+        if (st.second == nullptr)
+            return handle(); // no type info: error will be set already
 
         void *src_raw_void_ptr         = static_cast<void *>(src_raw_ptr);
         const detail::type_info *tinfo = st.second;
@@ -693,11 +696,13 @@ struct smart_holder_type_caster<std::unique_ptr<T, D>> : smart_holder_type_caste
             // SMART_HOLDER_WIP: IMPROVABLE: Error message.
             throw cast_error("Invalid return_value_policy for unique_ptr.");
         }
+        if (!src)
+            return none().release();
 
         auto src_raw_ptr = src.get();
         auto st          = type_caster_base<T>::src_and_type(src_raw_ptr);
-        if (st.first == nullptr)
-            return none().release(); // PyErr was set already.
+        if (st.second == nullptr)
+            return handle(); // no type info: error will be set already
 
         void *src_raw_void_ptr         = static_cast<void *>(src_raw_ptr);
         const detail::type_info *tinfo = st.second;