We were told (by the then-proto-Penitentiary-Major Mons. Luigi de Magistris) that it is illicit to withhold absolution on the condition of the penitent turning himself in to the civil authorities (even in cases of child abuse, murder, and the like). The priest can strongly recommend this, but the penance/absolution cannot be tied to this as a condition. The example that Mons. De Magistris used was if he were to hear the confession of a man on the eve of his ordination, who confessed having active homosexual relations, he would urge the young man to go to his rector and withdraw himself as a candidate for the priesthood and apply some other penance. If on the next day, that young man were ordained, Mons. De Magistris would go before him and kneel to request his priestly blessing as though nothing out of the ordinary had happened.
More about the seal of Confession at Fr. Z: A case about violation of the Seal of Confession in the press