// Common interface to all error custom types.
interface ESpeaker {
    IError
    speak() string
}

// One custom error implementation.
struct MyError {
    // Mandatory fields from IError.
    msg  string
    code int
    // Custom field.
    blah string
}

// Interface implementation for this example custom type.
fn (e MyError) speak() string {
    return e.blah
}

fn (e MyError) msg() string {
    return e.msg
}

fn (e MyError) code() int {
    return e.code
}

// An example function that returns a custom error.
fn foo() !string {
    return MyError{
        msg:  'foo'
        blah: 'world'
    }
}

fn test_interface_embedding_smartcast() {
    x := foo() or {
        if err is ESpeaker {
            err.speak()
        } else {
            'undefined'
        }
    }
    println(x)
    assert x == 'world'
}