hide attribute couldn't be overriden

[ajitjain]

if base mmd type has set hide="true" for a field then derived type is not able to change it to "false".
this is because inheritance handler is not able to determine (post-deserialization) whether the attribute actually didn't exist, or was "false". it then inherits the value from the base.

[quyin]

A workaround of this is to use always_show, which should take precedence over hide.

Fixing this will require changes in simpl. We will need a NullableBoolean type.