[dannnn]

It's not necessarily a "miscompile". C is supposed to be portable to even completely stupid architectures, and on some architectures, just loading an invalid pointer value into a register will cause a trap, even if you don't dereference it. So on those machines, if x is NULL, then an unoptimized compile of ((size_t)(&x->y)) will result in code that crashes. So that's why it's undefined.

(I'm pretty sure POSIX does impose certain non-stupidity requirements on its underlying architecture, so there are things that are undefined in plain C that are guaranteed to work on any POSIX platform. But I don't know if this is one of them.)