Problems with .isValid() on required checkboxes

Hey guys! I experienced some problems when making checkboxes required (for example terms and conditions).

See here for reference: http://jsfiddle.net/v6d5y3or/

Expected behaviour: Form .isValid() should reflect the checkbox value: checkbox.true = form.true and checkbox.false = form.false

Actual behaviour: Form .isValid() is always true: checkbox.true = form.true and checkbox.false = form.true

I'm not really sure why this happens and one would think that required: true works together with the checked binding.

A temporary solution to me is to implement a custom validator to make it work as expected:

ko.observable(false).extend({
  validation: [{
    validator: (value, params) => Boolean(value)
  }]
})

Start with the linked JSFiddle reproduction and trace how required validation handles Knockout checkbox checked bindings. Compare the unchecked and checked cases against the reported expected and actual isValid() results; done means an unchecked required checkbox makes the form invalid while a checked one makes it valid.