int (const 和 非const) 转换为 time.Duration

这是一个常见的操作，但是我们会遇到以下代码中的问题，甚是疑惑~

• 问题

      package main
      import (
          "fmt"
          "reflect"
          "time"
      )
      const a = 1
      func test(a time.Duration) {
          fmt.Printf("%v \n", a)
      }
      func main() {
          b := a + 1
          fmt.Printf("---- a = %v \n", reflect.TypeOf(a))
          test(a)
          fmt.Printf("---- b = %v \n", reflect.TypeOf(b))
          // test(b)  //> 打开注释后, 会编译不通过
      }
  

• 原因

  Assignability
      A value x is assignable to a variable of type T ("x is assignable to T") if one of the following conditions applies:
          - x's type is identical to T.
          - x's type V and T have identical underlying types and at least one of V or T is not a defined type.
          - T is an interface type and x implements T.
          - x is a bidirectional channel value, T is a channel type, x's type V and T have identical element types, and at least one of V or T is not a defined type.
          - x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type.
          - x is an untyped constant representable by a value of type T.  
  

就是说，在 const 修饰的变量如果没有明确指明类型时，会根据需要做类型转换; 而非 const 修饰的变量不会。这就可以解释上述代码中的疑惑了。

• reference: https://golang.org/ref/spec#Constants