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But this is a detail of implementation, user code does not use reflection with errors.

That "detail of implementation" is what is used by user code, e.g., `myError.As(...)` uses reflection, implicitly.

Then what? It has 0 performance impact, we're talking about error handling here, it's rare and not in hot path.

It does not have 0 performance impact, it's not rare, and in applications that are in the hot path, error handling is also in the hot path.

No, it only executes when you enter the err != nil block so it's almost never called, again 0 impact on performance.