Resulting from adoption, may be a matrimonial impediment, either impedient or diriment, depending on the nature of the civil law on the subject in the region where the marriage takes place.
If, according to that law, the marriage is valid although unlawful, the Church regards it in like manner, and the impediment is merely of the impedient or hindering class.
If the civil law holds that such a marriage is invalid, the Church makes the impediment a diriment one.
She may dispense from either.
New Catholic Dictionary