spec: constant value truncated to integer

[mattn]

Related issue #4565

println(int64(float64(7)/float64(3)+0.5))

This code become compile error. Using const is also.
http://play.golang.org/p/essxyc9aKX

package main

const (
    a = 7
    b = 3
)

func main() {
    println(int64(float64(a)/float64(b)+0.5))
}

But if the values are variables, it isn't.
http://play.golang.org/p/lHSeHRiG2G

a := 7.0
b := 3.0
println(int64(float64(a)/float64(b)+0.5))

I'm thinking, to express the expression, it's not useful to be error.

// r should be result of divides,
// (radius of the Earth) / (length of one step)
r := int(float64(RADIUS_OF_EARTH) / float64(LENGTH_OF_ONE_STEP))

[minux]

This is basically #6923.