cmd/compile: Ensure left-to-right assignment

Add temporaries to reorder the assignment for OAS2XXX nodes.
This makes orderstmt(), rewrite
  a, b, c = ...
as
  tmp1, tmp2, tmp3 = ...
  a, b, c = tmp1, tmp2, tmp3
and
  a, ok = ...
as
  t1, t2 = ...
  a  = t1
  ok = t2

Fixes #13433.

Change-Id: Id0f5956e3a254d0a6f4b89b5f7b0e055b1f0e21f
Reviewed-on: https://go-review.googlesource.com/34713
Run-TryBot: Dhananjay Nakrani <dhananjayn@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 37 insertions, 0 deletions

diff --git a/test/reorder.go b/test/reorder.go
index fc44be90de..3a87d025c2 100644
--- a/test/reorder.go
+++ b/test/reorder.go
@@ -19,6 +19,7 @@ func main() {
 	p6()
 	p7()
 	p8()
+	p9()
 }
 
 var gx []int
@@ -112,3 +113,39 @@ func p8() {
 		panic(m[0])
 	}
 }
+
+// Issue #13433: Left-to-right assignment of OAS2XXX nodes.
+func p9() {
+	var x bool
+
+	// OAS2FUNC
+	x, x = fn()
+	checkOAS2XXX(x, "x, x = fn()")
+
+	// OAS2RECV
+	var c = make(chan bool, 10)
+	c <- false
+	x, x = <-c
+	checkOAS2XXX(x, "x, x <-c")
+
+	// OAS2MAPR
+	var m = map[int]bool{0: false}
+	x, x = m[0]
+	checkOAS2XXX(x, "x, x = m[0]")
+
+	// OAS2DOTTYPE
+	var i interface{} = false
+	x, x = i.(bool)
+	checkOAS2XXX(x, "x, x = i.(bool)")
+}
+
+//go:noinline
+func fn() (bool, bool) { return false, true }
+
+// checks the order of OAS2XXX.
+func checkOAS2XXX(x bool, s string) {
+	if !x {
+		fmt.Printf("%s; got=(false); want=(true)\n", s)
+		panic("failed")
+	}
+}