Allow rvalue references of custom types

Author: rhaschke

include/pybind11/cast.h

@@ -644,6 +644,10 @@ struct copyable_holder_caster : public type_caster_base<type> {
     // static_cast works around compiler error with MSVC 17 and CUDA 10.2
     // see issue #2180
     explicit operator type&() { return *(static_cast<type *>(this->value)); }
+
+    // holders only support copying, not moving
+    template <typename T> using cast_op_type = detail::cast_op_type<T>;
+
     explicit operator holder_type*() { return std::addressof(holder); }
     explicit operator holder_type&() { return holder; }
 

include/pybind11/detail/type_caster_base.h

@@ -912,10 +912,12 @@ template <typename type> class type_caster_base : public type_caster_generic {
             nullptr, nullptr, holder);
     }
 
-    template <typename T> using cast_op_type = detail::cast_op_type<T>;
+    // allow moving of custom types
+    template <typename T> using cast_op_type = detail::movable_cast_op_type<T>;
 
     operator itype*() { return (type *) value; }
     operator itype&() { if (!value) throw reference_cast_error(); return *((itype *) value); }
+    operator itype&&() && { if (!value) throw reference_cast_error(); return std::move(*((itype *) value)); }
 
 protected:
     using Constructor = void *(*)(const void *);