Test order of evaluation in tuple assignments.

gccgo currently passes this test; 8g currently does not.  I
think I counted everything out right.

R=rsc
CC=golang-dev
https://golang.org/cl/186165

test/bugs/bug242.go

+// $G $D/$F.go && $L $F.$A && ./$A.out || echo BUG: tuple evaluation order
+
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test order of evaluation in tuple assignments.
+
+package main
+
+var i byte = 0
+var a [30]byte
+func f() *byte {
+	i++
+	return &a[i-1]
+}
+func gbyte() byte {
+	i++
+	return 'a' + i - 1
+}
+func gint() byte {
+	i++
+	return i - 1
+}
+func x() (byte, byte) {
+	i++
+	return 'a' + i - 1, 'a' + i - 1
+}
+func e1(c chan byte, expected byte) chan byte {
+	if i != expected {
+		panicln("e1: got", i, "expected", expected)
+	}
+	i++
+	return c
+}
+
+type Empty interface {}
+type I interface {
+	Get() byte
+}
+type S1 struct {
+	i byte
+}
+func (p S1) Get() byte {
+	return p.i
+}
+type S2 struct {
+	i byte
+}
+func e2(p Empty, expected byte) Empty {
+	if i != expected {
+		panicln("e2: got", i, "expected", expected)
+	}
+	i++
+	return p
+}
+func e3(p *I, expected byte) *I {
+	if i != expected {
+		panicln("e3: got", i, "expected", expected)
+	}
+	i++
+	return p
+}
+
+func main() {
+	for i := range a {
+		a[i] = ' '
+	}
+
+	*f(), *f(), *f() = gbyte(), gbyte(), gbyte()
+
+	*f(), *f() = x()
+
+	m := make(map[byte]byte)
+	m[10] = 'A'
+	var p1, p2 bool
+	*f(), p1 = m[gint()]
+	*f(), p2 = m[gint()]
+	if !p1 || p2 {
+		panicln("bad map check", i, p1, p2)
+	}
+
+	m[13] = 'B'
+	m[gint()] = gbyte(), false
+	if _, present := m[13]; present {
+		panicln("bad map removal")
+	}
+
+	c := make(chan byte, 1)
+	c <- 'C'
+	*f(), p1 = <-e1(c, 16)
+	*f(), p2 = <-e1(c, 18)
+	if !p1 || p2 {
+		panicln("bad chan check", i, p1, p2)
+	}
+
+	s1 := S1{'D'}
+	s2 := S2{'E'}
+	var iv I
+	*e3(&iv, 19), p1 = e2(s1, 20).(I)
+	*e3(&iv, 21), p2 = e2(s2, 22).(I)
+	if !p1 || p2 {
+		panicln("bad interface check", i, p1, p2)
+	}
+
+	s := string(a[0:i])
+	if s != "def   ii A     C       " {
+		panicln("bad array results:", s)
+	}
+}

test/golden.out

@@ -144,3 +144,8 @@ throw: interface conversion
 panic PC=xxx
 
 == bugs/
+
+=========== bugs/bug242.go
+bad map check 13 false false
+panic PC=xxx
+BUG: tuple evaluation order