Assess the current practice of some cities withholding information from the immigration authorities, and is there a 2nd or 3rd order effect (unintended consequences) in these decisions to withhold information?

Some cities, like San Francisco, known as "sanctuary cities" attempt to protect non-citizen people by not complying with ICE detainer requests or sending ICE additional information on a person who is known to be or suspected to be an undocumented person. The reality is, however, that being undocumented is not a criminal infraction: rather, it is a civil infraction. Local police, unless they are deputized to act as immigration enforcement by the bill 287(g) cannot arrest someone or hold them in jail simply for being undocumented.
Non-citizens absolutely have constitutional rights and it is unconstitutional, affirmed by several courts of this country, to hold undocumented people in jail or for ICE to apprehend them simply for being undocumented. Many undocumented people who are charged, but not convicted of a crime, are either held by local jails for ICE or are picked up by ICE once they post bail. Sanctuary cities are attempting to stop this unconstitutional and vicious practice. By preventing the detainment of undocumented immigrants, sanctuary cities attempt to keep undocumented and non-citizen families from being separated—rather than being scattered by the immigration system.