assertValid = (helper, candidate, passStyleOfRecur) => {
  helper.canBeValid(candidate, assertChecker);
  helper.assertRestValid(candidate, passStyleOfRecur);
}
The `assertRestValid` assumes that the `canBeValid` check has already passed.
Contexts where we cannot assume that should call `assertValid` instead,
which checks both conditions in the right order.

@param {PassStyleHelper} helper
@param {any} candidate
@param {(val: any) => PassStyle} passStyleOfRecur
@returns {void}