Dereference null is not always UB?

Question

I've always known for a fact that the Standard mandates dereferencing null is UB. However,

(Link 1) says

p = 0; *p; is not inherently an error.

and provides a link to

(Link 2) says

*p is not an error when p is null unless the lvalue is converted to an lvalue

(I believe it's a typo and probably should read lvalue is converted to an rvalue)

Link 1 also says

char* p = 0; char *q = &*(p)

is "not undefined", which I could only read as well-defined or at least implementation-defined

Can a language lawyer provide an authoritative explanation of what's going on?

Answer 1

I explored the topic of indirection through null pointers in this answer. In short, it is indeed well-defined per se, just as elaborated in your cited Core issues. The committee used the notion of an empty lvalue as proposed many years ago (but never adopted); *p is supposed to be such an empty lvalue, and unless we attempt to access the (non-existent) memory location behind that lvalue (for example, by performing an lvalue-to-rvalue conversion), all other operations behave as expected. E.g. &*p is equivalent to p, unless p is invalid. (This also works for pointers past-the-end of an array, which is necessary for the common idiom &arr[n]).

I also started drafting a paper for empty lvalues (this is WIP, and the rebase against N4640 isn't complete yet), so there's a chance we'll see more of this at a later stage.