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proposal: errors: Is(err, target) should allow err to decide even when target is nil #39167

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@pjebs

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@pjebs

Currently Is looks like this:

func Is(err, target error) bool {
	if target == nil {
		return err == target     // <-------------
	}
	isComparable := reflectlite.TypeOf(target).Comparable()
	for {
		if isComparable && err == target {
			return true
		}
		if x, ok := err.(interface{ Is(error) bool }); ok && x.Is(target) {
			return true
		}
		if err = Unwrap(err); err == nil {
			return false
		}
	}
}

See:
https://golang.org/src/errors/wrap.go?s=1170:1201#L29 and also
https://github.com/golang/xerrors/blob/master/wrap.go#L50

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