std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::insert_or_assign

1,4) If a key equivalent to k already exists in the container, assigns std::forward<M>(obj) to the mapped_type corresponding to the key k. If the key does not exist, inserts the new value as if by insert, constructing it from value_type(k, std::forward<M>(obj)).

2,5) Same as (1,4), except the mapped value is constructed from value_type(std::move(k), std::forward<M>(obj)).

3,6) If a key equivalent to k already exists in the container, assigns std::forward<M>(obj) to the mapped_type corresponding to the key k. If the key does not exist, constructs an object u of value_type with std::forward<K>(k), std::forward<M>(obj)), then inserts u into *this. If hash_function()(u.first) != hash_function()(k) || contains(u.first) is true, the behavior is undefined. The value_type must be EmplaceConstructible into unordered_map from std::forward<K>(k), std::forward<M>(obj). This overload participates in overload resolution only if Hash and KeyEqual are both transparent. This assumes that such Hash is callable with both K and Key type, and that the KeyEqual is transparent, which, together, allows calling this function without constructing an instance of Key.