lose() error { return nil }

    var v any = T{}

    switch v.(type) {
    case io.Reader:
        // ...
    case io.ReadCloser:
        // unreachable
    }

Even though \'T\' has a \'Close\' method and thus implements \'io.ReadCloser\',
\'io.Reader\' will always match first. The method set of \'io.Reader\' is a
subset of \'io.ReadCloser\'. Thus it is impossible to match the second
case without matching the first case.


Structurally equivalent interfaces

A special case of the previous example are structurally identical
interfaces. Given these declarations

    type T error
    type V error

    func doSomething() error {
        err, ok := doAnotherThing()
        if ok {
            return T(err)
        }

        return U(err)
    }

the following type switch will have an unreachable case clause:

    switch doSomething().(type) {
    case T:
        // ...
    case V:
        // unreachable
    }

\'T\' will always match before V because they are structurally equivalent
and therefore \'doSomething()\''s return value implements both.