I am aware of all these rules, but they don't address my question. ASAN and friends add instrumentation (at significant cost) at runtime and are imperfect, esp

I am aware of all these rules, but they don't address my question. ASAN and friends add instrumentation (at significant cost) at runtime and are imperfect, especially in multithreaded programs where accesses can race with frees. My question involved another thread (for example) freeing the pointer to an underlying allocation, which was previously in-bounds. In this case, whether it was UB to construct the pointer would depend upon whether the free was synchronized with the pointer arithmetic, which is not generally something compilers can work out in nontrivial programs (though these days this stuff is amenable to formal verification).