Golang 中一个 time.Duration 相关类型转换问题

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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。