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Yes. It won't compile.

This check is not really needed as the code would fail when a function which accepts scan.Writer would fail (compilation) if ColumnWriter is sent and if it doesn't satisfy the interface. However, this is easier to debug.

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Yeah, this type of check is nice if you intend to implement an interface, but don't actually use the type as that interface in your package. This will give a compile time error if your type doesn't satisfy the interface. You could write a test for it too, but this is a little simpler and harder to miss.

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