Set the true branch of the conditional.

template<
    typename T>
deferred_conditional< T, OnFalse > then(
    T on_true,
    typename constraint< is_deferred< T >::value >::type *  = 0,
    typename constraint< is_same< typename conditional< true, OnTrue, T >::type, deferred_noop >::value >::type *  = 0);