前往：命名类型断言和转换

If I have a custom type that simply redefines a pre-defined type with a name:

type Answer string

And I try to use it in a function that accepts the pre-defined type:

func acceptMe(str string) {
    fmt.Println(str)
}

func main() {
    type Answer string
    var ans Answer = "hello"

    // cannot use ans (type Answer) as type string in function argument
    acceptMe(ans)          
    // invalid type assertion: ans.(string) (non-interface type Answer on left)
    acceptMe(ans.(string)) 
    // Does work, but I don't understand why if the previous doesn't:
    acceptMe(string(ans))
}

Why does the type assertion fail, but the conversion work?

Type assertion works for interfaces only. Interface can have arbitrary underlying type, so we have type assertion and type switch to the rescue. Type assertion returns bool as the second return value to indicate if assertion was successful.

Your custom type Answer can have only one underlying type. You already know the exact type - Answer and the underlying type - string. You don't need assertions, since conversion to the underlying type will always be successful.

Old answer:

Just convert your custom type to string. The conversion will succeed since your custom type has string as an underlying type. The conversion syntax: string(ans). Go Play