A common rule here on PPCG is that programming languages invented after a challenge was posted cannot be used to answer that challenge. (I've had trouble finding a specific meta post that addresses this though.)
This rule makes total sense because without it, someone could "invent" a language that solves any single challenge, essentially automatically.
For example I could "invent" the language riCkroll for this challenge: riCkroll is identical to C except that the empty program outputs the lyrics to "Never Gonna Give You Up". (Modifying an existing compiler to do this would not be that hard.)
The problem I see with this rule is that there may be many perfectly valid languages, not based on loopholes, that were developed after a challenge was posted, and are unduly restricted. For example, I don't think any of us would find it particularly egregious if Mac posted his "99 Bottles of Beer" program to this challenge in my new 99 language (invented yesterday). I agree that it shouldn't be allowed to win (be the accepted answer), just so we can avoid the loopholes mentioned above, but there seems no harm in allowing it to be be posted.
So I'm voting we change the rule of
Languages that were developed after the challenge was posted are not allowed.
Languages that were developed after the challenge was posted are not allowed to win.
Do you agree?