Frank Mori Hess:
Yes, I think it does break get_deleter(), I hadn't considered that. It's done because all the shared_ptrs created by shared_from_this() before the object is owned by the user's shared_ptr need to be made consistent with the shared_count in the user's shared_ptr. The ethical course would have been to take the shared_count from the user's shared_ptr and copy it into any preexisting shared_ptrs created by shared_from_this(). However, there may be many of those, and there was no list of references to them available. So I took the practical course, and went the opposite direction by copying a shared_count from the preexisting shared_ptrs into the user's shared_ptr (of which there is only one, and which has a handy reference available).
My current line of thinking is that we should refrain from introducing regressions. After shared_ptr<X> px( new X, d ); px should contain the deleter d and - in the case when shared_from_this hasn't been called in the constructor - px.use_count() should be 1. Is it really that important to make the shared_ptr instances created in X::X share ownership with px?