It's hypothetical because you yourself admit the standard library implementations don't actually do this here. The real C++ string contains() does in fact only work for the three specific types implemented, and is not generic, and that's not merely an oversight it's how it necessarily works even though it's worse. Circle's Interfaces would offer a much nicer approach, which makes sense because they do the same thing as Rust's traits for this scenario.

You're right that in some places C++ does the generic thing, I pointed at std::sort already - there are lots of examples of varying quality. The way we got here is that I pointed out overloads are the Wrong Thing™ and that's why it makes sense Carbon would want to outlaw them, and why no, outlawing them does not lose the nice property where the API feels generic - you can do parametric polymorphism and still have the generic API.

The main thing outlawing overloading gets rid of is actually functions/ methods which quietly have more than one distinct purpose. These functions are foot guns. Use your words, call the two (or more) things by different names reflecting the difference in intent and then fewer developers will mistakenly invoke the wrong meaning.

I don't like ADL either, but it's pretty far off topic.