For unordered associative containers, if an exception is thrown by any
operation other than the container's hash function from within an
insert or emplace function inserting a single element,
the insertion has no effect.
For unordered associative containers, no swap function throws
an exception unless that exception is thrown by the swap of the container's
Hash or Pred object (if any).
For unordered associative containers, if an exception is thrown
from within a rehash() function other than by the container's hash
function or comparison function, the rehash() function has no effect.