What type of law punishes people for doing something that was once legal but is illegal?

Answer 1

It's called an "ex post facto" law. "Es post facto" is latin for "after the fact."

In the U.S., and most other developed nations, such laws are constitutionally prohibited, because they're viewed as manifestly unfair. Most would agree that it's absurd to punish a person for an act that was legal at the time the act was committed.

Accordingly, all laws must be prospective (applying forward from the time of enactment), rather than retroactive (applying backward from the time of enactment).

Answer 2

An ex post facto law (from the Latin for "from after the action") or retroactive law is a law that retroactively changes the legal consequences (or status) of actions committed or relationships that existed prior to the enactment of the law. In reference to criminal law, it may criminalize actions that were legal when committed; or it may aggravate a crime by bringing it into a more severe category than it was in at the time it was committed; or it may change or increase the punishment prescribed for a crime, such as by adding new penalties or extending terms; or it may alter the rules of evidence in order to make conviction for a crime more likely than it would have been at the time of the action for which a defendant is prosecuted. Conversely, a form of ex post facto law commonly known as an amnesty law may decriminalize certain acts or alleviate possible punishments (for example by replacing the death sentence with life-long imprisonment) retroactively.

Answer 3

Those are called ex post facto laws, and they are specifically prohibited by the US Constitution.

Answer 4

That would be simply a new law or current law.

Answer 5

ex post facto

Answer 6

Ex Post Facto