Domain restriction doesn't work with selection widget

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yanked from openobject-client-gtk:

+++
many2one field with a 'widget="selection" attribute in the XML view
definition. Then any domain restriction won't be applied in neither of the two
following cases:

CASE 1

PY model definition:
'country_id': field.many2one('res.country', 'Country'),
'state_id': field.many2one('res.country.state', 'State',
domain="[('country_id','=',country_id)]"),

XML view definition:
<field name="country_id" widget="selection" />
<field name="state_id" widget="selection" />

CASE 2

PY model definition:
'country_id': field.many2one('res.country', 'Country'),
'state_id': field.many2one('res.country.state', 'State'),

XML view definition:
<field name="country_id" widget="selection" />
<field name="state_id" widget="selection"
domain="[('country_id','=',country_id)]" />