The US Supreme Court has repeatedly held the opposite - that police officers are immune to prosecution for their ignorance of the bounds of their powers unless explicitly informed.
IANAL; I believe you are referring to the second prong of the qualified immunity (QI): "right infringed has to be "clearly established" at the time of the official's conduct."
We should maintain QI but this second test needs an update.
We should maintain QI but this second test needs an update.