Proposed resolution:

This wording is relative to N4659.

1. Edit [util.smartptr.shared.cast] as indicated:

   shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;
   

   -4- Requires: The expression dynamic_cast<T*>((U*)0) shall be well formed. The expression dynamic_cast<typename shared_ptr<T>::element_type*>(r.get()) shall be well formed and shall have well defined behavior.

   […]

[ 2017-07 Toronto Monday issue prioritization ]

Priority 0; move to Ready

Currently [util.smartptr.shared.cast]/4 says:

Requires: The expression dynamic_cast<T*>((U*)nullptr) shall be well formed and shall have well defined behavior.

A dynamic_cast of a null pointer, if well-formed, always has well-defined behavior: it returns a null pointer. The second part is therefore redundant as currently worded. The C++14 version, on the other hand, requires dynamic_cast<T*>(r.get()) to have well-defined behavior, which actually adds something: it requires the user to not trigger the undefined case in [class.cdtor]/5, for instance.