A pity the c++ compiler has no way to recognise calls to the c++ library in order to do rewrites like that automatically. It's CSE at the stdlib level, should definitely be possible to do that.

Hard coding behavior like for the STL that seems pretty questionable, especially given that std::map and std::unordered_map have poor performance compared to other alternatives (e.g. absl::btree_map and absl::flat_hash_map, and likewise folly has better implementations).

if let is such a nice way to express this

    if let Some(x) = map.get(foo) {
        bob(x)
    }

One subtle but noteworthy thing is that the idiom only mentions map by name once, which is nice if you have a very long map name, and prevents copy paste bugs where you only update one of the two mentions.

Scala:

    for (x <- map.get(foo)) {
      bob(x)
    }

    // or

    map.get(foo).foreach(bob)

    // or

    map.get(foo) match {
      case Some(x) => bob(x)
    }

This pattern exists in C++ as well. The specific issue here is that all these STL APIs are in terms of C++'s iterator model, and can't be replaced with a more modern "optional" style that allows this cleaner coding style :-/