This avoids a value copy when calling value() or value_or() on a
temporary Optional. This is very common when using the HashMap::get()
API like this:
auto value = hash_map.get(key).value_or(fallback_value);
This patch introduces a new operator== to compare an Optional to its
contained type directly. If the Optional does not contain a value, the
comparison will always return false.
This also adds a test case for the new behavior as well as comparison
between Optional objects themselves.
