神秘类型断言失败？

Question

Under what circumstances could this code:

        v, ok := value.(int64)
        if !ok  {
            panic("NOPE "+reflect.TypeOf(value).Kind().String())
        } else {
            fmt.Printf("VAL: %d
",v)
        }

produce the panic with message panic: NOPE int64?

Is this a bug or is there something basic about the numeric types that I'm missing?

Answer 1

This can happen if you're using type declaration on numeric types. If you do something like this:

type T int64 

...

var value interface{} = T(1)

and put it into your code, you'll get the exact same error. But if you don't check the kind, but the type, you'll see what's happening here:

v, ok := value.(int64)
if !ok {
    panic("NOPE " + reflect.TypeOf(value).String())
} else {
    fmt.Printf("VAL: %d
", v)
}

produces the message:

panic: NOPE main.T

The kind of T is int64, but value is not int64.