interp: handle getting address of interpreter interface value

Fixes #963.

_test/addr2.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"encoding/xml"
+	"fmt"
+)
+
+type Email struct {
+	Where string `xml:"where,attr"`
+	Addr  string
+}
+
+func f(s string, r interface{}) error {
+	return xml.Unmarshal([]byte(s), &r)
+}
+
+func main() {
+	data := `
+		<Email where='work'>
+			<Addr>bob@work.com</Addr>
+		</Email>
+	`
+	v := Email{}
+	err := f(data, &v)
+	fmt.Println(err, v)
+}
+
+// Ouput:
+// <nil> {work bob@work.com}

interp/run.go

@@ -588,12 +588,23 @@ func not(n *node) {
 
 func addr(n *node) {
 	dest := genValue(n)
-	value := genValue(n.child[0])
 	next := getExec(n.tnext)
-
-	n.exec = func(f *frame) bltn {
-		dest(f).Set(value(f).Addr())
-		return next
+	c0 := n.child[0]
+	value := genValue(c0)
+	switch c0.typ.cat {
+	case interfaceT:
+		i := n.findex
+		l := n.level
+		n.exec = func(f *frame) bltn {
+			v := value(f).Interface().(valueInterface).value
+			getFrame(f, l).data[i] = reflect.ValueOf(v.Interface())
+			return next
+		}
+	default:
+		n.exec = func(f *frame) bltn {
+			dest(f).Set(value(f).Addr())
+			return next
+		}
 	}
 }