Fix remaining issues

To ensure constness of reference arguments passed to overridden functions,
these should be copied instead of referenced.
However, this doesn't apply to const unique_ptr&, which should be referenced
in any case. To this end, introduce a new return_value_policy: reference_override

Author: rhaschke

include/pybind11/detail/common.h

@@ -434,7 +434,14 @@ enum class return_value_policy : uint8_t {
         collected while Python is still using the child. More advanced
         variations of this scheme are also possible using combinations of
         return_value_policy::reference and the keep_alive call policy */
-    reference_internal
+    reference_internal,
+
+    /** This internally used policy only applies to C++ arguments passed
+        to virtual methods overridden in Python. This effectively will result
+        in the return_value_policy::reference policy usually.
+        It is needed to suppress copying of const unique_ptr, which is
+        otherwise applied to const refererences to ensure constness. */
+    reference_override
 };
 
 PYBIND11_NAMESPACE_BEGIN(detail)

include/pybind11/detail/smart_holder_type_casters.h

@@ -474,15 +474,23 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
     static handle cast(T const &src, return_value_policy policy, handle parent) {
         // type_caster_base BEGIN
         // clang-format off
-        if (policy == return_value_policy::automatic || policy == return_value_policy::automatic_reference)
-            policy = return_value_policy::copy;
-        return cast(&src, policy, parent);
+        if (policy == return_value_policy::automatic ||
+            policy == return_value_policy::automatic_reference ||
+            policy == return_value_policy::reference_override)
+            policy = return_value_policy::copy; // copy ensures constness
         // clang-format on
+        return cast(&src, policy, parent);
         // type_caster_base END
     }
 
     static handle cast(T &src, return_value_policy policy, handle parent) {
-        return cast(const_cast<T const &>(src), policy, parent); // Mutbl2Const
+        // type_caster_base BEGIN
+        // clang-format off
+        if (policy == return_value_policy::automatic || policy == return_value_policy::automatic_reference)
+            policy = return_value_policy::reference;
+        return cast(&src, policy, parent);
+        // clang-format on
+        // type_caster_base END
     }
 
     static handle cast(T const *src, return_value_policy policy, handle parent) {
@@ -559,6 +567,7 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
 
             case return_value_policy::automatic_reference:
             case return_value_policy::reference:
+            case return_value_policy::reference_override:
                 valueptr       = src;
                 wrapper->owned = false;
                 break;
@@ -723,7 +732,9 @@ struct smart_holder_type_caster<std::unique_ptr<T, D>> : smart_holder_type_caste
             return none().release();
         if (policy == return_value_policy::automatic)
             policy = return_value_policy::reference_internal;
-        if (policy != return_value_policy::reference_internal)
+        else if (policy == return_value_policy::reference_override)
+            policy = return_value_policy::reference;
+        else if (policy != return_value_policy::reference_internal)
             throw cast_error("Invalid return_value_policy for unique_ptr&");
         return smart_holder_type_caster<T>::cast(src.get(), policy, parent);
     }

include/pybind11/detail/type_caster_base.h

@@ -527,6 +527,7 @@ class type_caster_generic {
 
             case return_value_policy::automatic_reference:
             case return_value_policy::reference:
+            case return_value_policy::reference_override:
                 valueptr = src;
                 wrapper->owned = false;
                 break;

include/pybind11/eigen.h

@@ -303,6 +303,7 @@ struct type_caster<Type, enable_if_t<is_eigen_dense_plain<Type>::value>> {
                 return eigen_array_cast<props>(*src);
             case return_value_policy::reference:
             case return_value_policy::automatic_reference:
+            case return_value_policy::reference_override:
                 return eigen_ref_array<props>(*src);
             case return_value_policy::reference_internal:
                 return eigen_ref_array<props>(*src, parent);

include/pybind11/pytypes.h

@@ -105,9 +105,9 @@ class object_api : public pyobject_tag {
         function will throw a `cast_error` exception. When the Python function
         call fails, a `error_already_set` exception is thrown.
     \endrst */
-    template <return_value_policy policy = return_value_policy::reference, typename... Args>
+    template <return_value_policy policy = return_value_policy::reference_override, typename... Args>
     object operator()(Args &&...args) const;
-    template <return_value_policy policy = return_value_policy::reference, typename... Args>
+    template <return_value_policy policy = return_value_policy::reference_override, typename... Args>
     PYBIND11_DEPRECATED("call(...) was deprecated in favor of operator()(...)")
         object call(Args&&... args) const;