Proposed resolution:

This wording is relative to N4659.

1. In [unord.req], edit Table 91 "Unordered associative container requirements" as indicated:

   Table 91 — Unordered associative container requirements (in addition to container)

   Expression	Return type	Assertion/note pre-/post-condition	Complexity
   …
   a.merge(a2)	void	Requires: a.get_allocator() == a2.get_allocator(). Attempts to extract each element in a2 and insert it into a using the hash function and key equality predicate of a. In containers with unique keys, if there is an element in a with key equivalent to the key of an element from a2, then that element is not extracted from a2. Postconditions: Pointers and references to the transferred elements of a2 refer to those same elements but as members of a. Iterators referring to the transferred elements and all iterators referring to a will be invalidated, but iterators to elements remaining in a2 will remain valid. Throws: Nothing unless the hash function or key equality predicate throws.	Average case 𝒪(N), where N is a2.size(). Worst case 𝒪(N*a.size()+N).
   …

[ 2017-07 Toronto Monday issue prioritization ]

Priority 0; move to Ready

As pointed out in this StackOverflow question, unordered_{map,multimap,set,multiset}::merge() may need to rehash to maintain its max_load_factor invariant, which may require allocation, which may throw.