Why are self-referential struct parameters impossible?

Briefly looking at the issue, I suspect we’ve run into the deepest reason of all: insufficient brain clockcycles allocated to the problem. Maybe you’re the type to have the skills and motivation to help solve it?

Since no instances of the struct have been instantiated yet (as errors would be thrown if you try to instantiate an incompletely defined struct), what do you think would be the hardest part(s)?