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.

concepts have been excellent for me for overload resolution in complex template code (good riddance SFINAE) and for documentation.

To improve error messages, not so much.