此类型断言曾经有效吗？

I am reading the source code for Go's io package and I came across a snippet I don't fully understand. Here it is

func WriteString(w Writer, s string) (n int, err error) {
    if sw, ok := w.(stringWriter); ok {
        return sw.WriteString(s)
    }
    return w.Write([]byte(s))
}

where

type stringWriter interface {
    WriteString(s string) (n int, err error)
}

type Writer interface {
    Write(p []byte) (n int, err error)
}

The type assertion w.(stringWriter) asserts that the dynamic type of w (i.e. Writer) implements the stringWriter interface. I don't see how this is possible given the type definitions of stringWriter and Writer. Assuming that this code is correct, what is it that I am missing?

You can easily build a type which implements Writer as well as stringWriter:

type StringWriter struct{}

func (s StringWriter) Write(in []byte) (int, error) { 
    return 0, nil 
}
func (s StringWriter) WriteString(s string) (int, error) { 
    return s.Write([]byte(s)) 
}

So passing an instance of StringWriter to io.WriteString results in StringWriter.WriteString being called.

The logic behind io.WriteString is: Use WriteString on the writer if it has such a method or fall back to string to []byte conversion otherwise. To test whether the instance implements a method or not, interfaces are used, as these are just method sets and can be tested for easily.