1. What is a short input program that triggers the error?
// should print {0 300} but prints {0 -500} instead.
// changing Bbox to a value method fixes it. as does splitting
// the calculation up.

package main

import (
    "exp/draw"
    "fmt"
)

type Obj struct {
    bbox draw.Rectangle
}

func (o *Obj) Bbox() draw.Rectangle {
    return o.bbox
}

func main() {
    obj := &Obj{draw.Rect(800, 0, 0, 0)}
    p := draw.Pt(800, 300)
    fmt.Printf("%v\n", calc(obj, p))
}


func calc(obj *Obj, p draw.Point) draw.Point {
    return p.Sub(obj.Bbox().Min)
}



2. What is the full compiler output?
it compiles ok.

3. What version of the compiler are you using?  (Run it with the -V flag.)
8g version 5516+