As checks if err is a *Warning, and returns it.

Is reports if err is a *Warning.

New creates a new warning that wraps one or more errors.

Newf creates a new warning that wraps a single error created with fmt.Errorf.

Wrap returns a new warning with no message that wraps one or more errors.

Wrapf wraps a single error in a warning, with a message created using a format string.

Warning is a kind of error that exists so that parsing/processing functions can produce warnings that do not abort part-way, but can still be reported via the error interface and logged.