Select widget filter accepts a default value that is not part of the options

[maximlt]

The following filter's spec, defined in the source spec, doesn't raise any error even if the default isn't part of the options (obtained from the schema).

    filters:
      species:
        type: widget
        field: species
        multi: false
        default: bad

Before any value is selected in the UI, the filter has a value='bad' and the widget has a correct value (i.e. one part of its options).

This is ultimately down to panel, where the Select widget doesn't raise any error if one tries to set a value not part of the options.

import panel as pn
from panel.widgets import Select
s = Select(value='b', options=['a', 'b'])
print(s.value)  # 'b'
s.value = 'c'
print(s.value)  # 'a'

Instead panel just sets the first the new value to the first option available (https://github.com/holoviz/panel/blob/master/panel/widgets/select.py#L73).

@philippjfr why does panel have this behavior? I'd expect an error to be raised (as with param.Selector) but maybe there's a good reason for not following param here.