title: Clarification and reasons for object lifetime constraints change in C++20
body: <p>As of C++20, there's a significant change for the constraints of object lifetime from <a href="https://timsong-cpp.github.io/cppwp/n4659/basic.life#8.3" rel="nofollow noreferrer">basic.life#8.3</a> to
  <a href="https://timsong-cpp.github.io/cppwp/n4861/basic.life#8.3" rel="nofollow noreferrer">n4861/basic.life#8.3</a>. The concrete change I want to focus on here is (C++20 draft)</p>
  <blockquote>
  <p>If, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, a new object is created at the storage location which the original object occupied, a pointer that pointed to the original object, a reference that referred to the original object, or the name of the original object will automatically refer to the new object and, once the lifetime of the new object has started, can be used to manipulate the new object, if the original object is transparently replaceable (see below) by the new object. An object o1 is transparently replaceable by an object o2 if</p>
  <ol>
  <li>the storage that o2 occupies exactly overlays the storage that o1 occupied, and</li>
  <li>o1 and o2 are of the same type (ignoring the top-level cv-qualifiers), and</li>
  <li><strong>o1 is not a complete const object</strong>, and</li>
  <li>neither o1 nor o2 is a potentially-overlapping subobject, and</li>
  <li>either o1 and o2 are both complete objects, or o1 and o2 are direct subobjects of objects p1 and p2, respectively, and p1 is
  transparently replaceable by p2.</li>
  </ol>
  </blockquote>
  <p>vs. the pre-C++20 one (not the only change, see the drafts for details)</p>
  <blockquote>
  <ol start="3">
  <li><strong>the type of the original object is not const-qualified, and, if a
  class type, does not contain any non-static data member whose type is
  const-qualified or a reference type...</strong></li>
  </ol>
  </blockquote>
  <p>Question 1:
  Maybe too obvious, just to be sure: With <code>complete const object</code>, the standard refers to complete objects that are const, right? (So it has nothing to do with partial/full const objects that was part of the previous quality of wording within this section?)</p>
  <p>Question 2:
  Can anyone explain the reasons behind these changes (aliasing?)?</p>
  <p>Question 3:
  Is my assumption valid, that this one is a heavy relaxation of the former rules, i.e. guaranteeing now a lot more object/memory reusage scenarios not to be UB for instance? In doubt, shouldn't they affect the way the optimizers are allowed to operate in a heavy way too (shift of efficiency fields)?</p>