[line-zero]

  *In the United States, the Congress is prohibited from passing ex post facto laws by clause 3 of Article I, Section 9 of the United States Constitution.*

I don't know if you've noticed, but folks in Washington don't give a fuck about the Constitution any longer.

[zodiac]

But if that hypothetical law were passed by Congress, people sued under that law would argue that the law was unconstitutional, and any competent court would agree, acquit the person, and strike down the law.