This type of runtime type checking is quite common in Go. An example from bufio.Reader's WriteTo method:
// WriteTo implements io.WriterTo.
// This may make multiple calls to the [Reader.Read] method of the underlying [Reader].
// If the underlying reader supports the [Reader.WriteTo] method,
// this calls the underlying [Reader.WriteTo] without buffering.
func (b *Reader) WriteTo(w io.Writer) (n int64, err error) {
b.lastByte = -1
b.lastRuneSize = -1
n, err = b.writeBuf(w)
if err != nil {
return
}
if r, ok := b.rd.(io.WriterTo); ok {
m, err := r.WriteTo(w)
n += m
return n, err
}
if w, ok := w.(io.ReaderFrom); ok {
m, err := w.ReadFrom(b.rd)
n += m
return n, err
}
if b.w-b.r < len(b.buf) {
b.fill() // buffer not full
}
for b.r < b.w {
// b.r < b.w => buffer is not empty
m, err := b.writeBuf(w)
n += m
if err != nil {
return n, err
}
b.fill() // buffer is empty
}
if b.err == io.EOF {
b.err = nil
}
return n, b.readErr()
}