If you have an interface which indicates that some properties must be G_PARAM_READABLE, it is a common GObject pattern to add a G_PARAM_WRITABLE bit to a class implementing that property (when it makes sense).

ValaProperty incorrectly assumes that the base interface not having a set accessor means that the property is invalid and errors as such:

  Dazzle-1.0.gir:561.7-561.33: error: Type and/or accessors of overriding property `Dzl.ChildPropertyAction.enabled' do not match overridden property `GLib.Action.enabled': incompatible set accessor.

One temporary workaround is to just add a function for the setter, but that breaks the ability to use GBinding.

I assume this is also broken for G_PARAM_WRITABLE properties in an interface (where the class implementation adds a G_PARAM_READABLE bit).