This is a good rule of thumb, mostly because its true. I'm not sure why people have trouble understanding this (poor civics classes? too dense?) but the US govt. can do anything it physically can. Sure, it may require a constitutional amendment, but if the political will is there the govt. can do anything.

But even in softer formulations, Congress can use general welfare to get away with anything SCOTUS will let them get away with.

Luckily though, unlike everyone else on Hacker News, I actually read the Constitution and thus happen to know this line:

To coin Money, regulate the Value thereof, and of foreign Coin, and fix the Standard of Weights and Measures;