class_: Register reclaim_from_cpp with constructing type

include/pybind11/pybind11.h

@@ -1392,11 +1392,17 @@ class class_ : public detail::generic_type {
         auto* tinfo = get_type_info();
         // TODO(eric.cousineau): Should relax this to not require a holder be constructed,
         // only that the holder itself be default (unique_ptr<>).
-        if (inst->owned || v_h.holder_constructed()) {
-            throw std::runtime_error("Derived Python object should live in C++");
+        if (inst->owned) {
+            throw std::runtime_error(
+                "reclaim_from_cpp: Python object already owned! Did you forget to explicitly use a "
+                "py::return_value_policy (e.g. reference or reference internal) when passing back "
+                "non-owned pointers of the C++ object?");
+        }
+        if (v_h.holder_constructed()) {
+            throw std::runtime_error("reclaim_from_cpp: Holder already exists - internal error?");
         }
         if (!external_holder_raw) {
-            throw std::runtime_error("Internal error - not external holder?");
+            throw std::runtime_error("reclaim_from_cpp: No external holder - internal error?");
         }
         // Is this valid?
         handle h(reinterpret_cast<PyObject*>(inst));
@@ -1425,7 +1431,7 @@ class class_ : public detail::generic_type {
                 break;
             }
             default: {
-                throw std::runtime_error("Unsupported load type");
+                throw std::runtime_error("reclaim_from_cpp: Unsupported load type");
             }
         }
         inst->owned = true;
@@ -1716,6 +1722,9 @@ class class_ : public detail::generic_type {
         // method? (Rather than a class method?)
         object del_orig = getattr(h_type, "__del__", none());
 
+        // Set reclaim method.
+        inst->reclaim_from_cpp = reclaim_from_cpp;
+
 #if defined(PYPY_VERSION)
         // PyPy will not execute an arbitrarily-added `__del__` method.
         // Later version of PyPy throw an error if this happens.