If the the OP is the author, did consider filing a bug with errcheck? It should be possible for errcheck to check if the comparison is being done within an `Is(err error) bool` method and skip the warning in that case, or even better: it could check if your using `errors.Is` within an `Is` method and warn in that case!

The linter in the post is `err113`. `errortype` does already warn:

https://github.com/fillmore-labs/errortype#errortype