time.Millisecond * int混乱

Question

In the below example, if the 1000's are both int's (which I think they are) why would the bottom fail to compile?

//works
time.Sleep(1000 * time.Millisecond)

//fails
var i = 1000
time.Sleep(i * time.Millisecond)

Answer 1

Operators

Operators combine operands into expressions.

Comparisons are discussed elsewhere. For other binary operators, the operand types must be identical unless the operation involves shifts or untyped constants. For operations involving constants only, see the section on constant expressions.

Except for shift operations, if one operand is an untyped constant and the other operand is not, the constant is converted to the type of the other operand.

For example, using the "*" (multiplication) operator,

package main

import (
    "time"
)

func main() {

    // works - 1000 is an untyped constant
    // which is converted to type time.Duration
    time.Sleep(1000 * time.Millisecond)

    // fails - v is a variable of type int
    // which is not identical to type time.Duration
    var v = 1000
    // invalid operation: i * time.Millisecond (mismatched types int and time.Duration)
    time.Sleep(v * time.Millisecond)
}