How to set a value in an unordered_map and find out if a new key was added

How can I efficiently and idiomatically set a value in an unordered_map and find out if a new key was added:

#include <unordered_map>
#include <string>

int main() {
  auto map = std::unordered_map<std::string, int>{{"foo", 1}, {"bar", 2}};

  map["foo"] = 3;
  // how to find out if a new key was added?
}

I can't use insert() because I want to overwrite the value if there is one already and insert does not do that. I can't use operator[] directly because it provides no information about whether a new key was added.

I want to avoid doing two searches in the map for performance reasons.

One trick I've seen elsewhere is to get a reference and check if that value is default constructed:

auto& value = map["foo"];
if(value == 0) {
    // am inserting a new key
}
value = 3;

But I can genuinely have default constructed values in my map so a default constructed value is not a good indication of new key.

The best I can come up with so far is:

auto size_before = map.size();
map["foo"] = 3;
if (map.size() > size_before) {
    // am inserting a new key
}

which seems ugly and it assumes that getting the size of an unordered_map is cheap (is it?).

It looks like unordered_map::insert_or_assign might be the answer to my prayers but sadly it comes in C++17 so I probably wont be able to use it for another 5 years or so. It seems like quite a common thing to want to do so I assume there must be a reasonable way of doing it currently.