C++20 added concepts as a language feature. They’re often compared to Haskell’s type classes, Rust’s traits or Swift’s protocols. Yet there is one feature that sets them apart: types model C++ concepts automatically. In Haskell, you need an instance, in Rust, you need an impl, and in Swift, you need an extension. But in C++? In C++, concepts are just fancy boolean predicates that check for well-formed syntax: every type that makes the syntax well-formed passes the predicate and thus m...
