[deschutes]

I think the dark side of it is unintentional ADL but it's mostly good. C++ gets a lot of mileage out of concepts in templates. The newer language feature just makes the existing practice first class as something a bit beyond syntactic sugar. But the design of the STL relies heavily on the idea that if something has the requisite properties it is that thing. Iterators are probably the most prominent example. You won't find any explicit iterator types. It's just a set of properties that a type has that makes it an iterator or not. Personally I've never seen confusion be an issue. Granted the iterator and algorithm design has been mostly superceded by newer apis that operate as pipes of functions over streams. Frankly the experience of these are poor in C++ for various reasons.

One big example is that keys in associative collections are const qualified even when moving from the collection. The constness doesn't match the expectation users have when consuming a collection and is unfixable within the constraints of the STL. Anyway it results in awful type checking errors. The whole library is full of these foot guns and most of them result in bizarre behavior or horrible error messages.

Iterators have their own warts but IMO work much better within the C++ type system. Here's a fun one. Reverse iterators have their own set of invalidation properties which are typically weaker and different than forward iterators. Due to various reasons they actually refer to the element that precedes (in reverse sequence) the one you'll get when dereferencing it. So end is rfront and front is rend.

In either case the experience is quite bad compared to the stream apis you get from rust. But I don't think this is a mark against concepts, just a dated design and the limits of the type system and semantics of the language.