问理解go中类型转换的困惑

EN

    package main

    import (
    "fmt"
    )

    type val []byte

   func main() {
    var a []byte = []byte{0x01,0x02}
    var b val = a
    fmt.Println(a)
    fmt.Println(b)
}

package main

import (
    "fmt"
)

type abc string

func main() {
    fm := fmt.Println
    var second = "whowww"
    var third abc = second //compile Error at this line 12
    fm(second)
    fm(third)

}