P2786R13 defines replaceability as the equivalence between move
assignment, and destruction + move construction. While it conceptually
holds for many types, assignment usually has to be careful around
possible aliasing. For example, the paper calls out std::shared_ptr as
being replaceable, but consider:

    struct X {
        std::shared_ptr<X> x;
    };

    int main () {
        std::shared_ptr<X> x = std::make_shared<X> ();

        x->x = std::make_shared<X> ();

        x = std::move (x); // (1)
        x = std::move (x->x); // (2)
    }

Neither (1) nor (2) can be substituted for destroy + reconstruct.

It seems like the definition of replaceability should somehow exclude
those cases, or else many types become non-replaceable.