Proposed resolution:

This wording is relative to N5046.

1. Modify [unord.map.elem] as indicated:

   constexpr mapped_type& at(const key_type& k);
   constexpr const mapped_type& at(const key_type& k) const;
   

   -5- Returns: A reference to `x.second`, where `x is the (unique) element whose key is equivalent to `k`.

   -6- Throws: An exception object of type `out_of_range` if no such element is present.

   -?- Complexity: Average case constant, worst case linear in `size()`.

   template<class K> constexpr mapped_type&       at(const K& k);
   template<class K> constexpr const mapped_type& at(const K& k) const;
   

   -7- Constraints: The qualified-ids `Hash::is_transparent` and `Pred::is_transparent` are valid and denote types.

   -8- Preconditions: The expression `find(k)` is well-formed and has well-defined behavior.

   -9- Returns: A reference to `find(k)->second`.

   -10- Throws: An exception object of type `out_of_range` if `find(k) == end()` is `true`.

   -?- Complexity: Average case constant, worst case linear in `size()`.

During review of P3091R5 Pablo pointed out that there is no Complexity: element for `unordered_map::at`.