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Concepts support (like traits I'm Rust) is necessary for good diagnostics, but not sufficient. It gives you the architecture and metadata that you require to infer what the user wants, but a lot of additional analysis on the decision chain that was followed in order to figure out what is relevant and what isn't.

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Also the C++ 20 "Concepts Lite" isn't a nominal system like Rust's traits (or like the C++ 0x Concepts proposal which Bjarne had ripped out), it's Duck typing again and so it's harder even if the work was done.

Rust can say hey, this code treats Mallard as if it's a Duck, did you mean for this Mallard to implement Duck? If so, try writing #[derive(Duck)]

But for Concepts Lite the compiler needs to guess that you wanted Mallard to match this requirement that Ducks can quack, and observe that Mallard's quack has a slightly wrong signature so it doesn't match and so that's probably the mistake.

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