> and is no longer undefined behaviour to act on them
This is only partially true. From the proposal [0]:
> Moreover, a variable that has a well-defined, erroneous initial object representation under this proposal can still lead to undefined behaviour later on:
void f(T&);
void g(bool);
void h() {
T* p; // uninitialized, has erroneous value (e.g. null)
bool b; // uninitialized, erroneous value may not be valid bool
f(*p); // dereferencing has undefined behaviour
g(b); // undefined behaviour if b is invalid
}