cmd/compile: fix select statement evaluation order corner case

The Go spec requires that select case clauses be evaluated in order,
which is stricter than normal ordering semantics. cmd/compile handled
this correctly for send clauses, but was not correctly handling
receive clauses that involved bare variable references.

Discovered with @cuonglm.

Fixes #43111.

Change-Id: Iec93b6514dd771875b084ba49c15d7f4531b4a6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/277132
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/order.go

@@ -891,7 +891,7 @@ func (o *Order) stmt(n *Node) {
 				// c is always evaluated; x and ok are only evaluated when assigned.
 				r.Right.Left = o.expr(r.Right.Left, nil)
 
-				if r.Right.Left.Op != ONAME {
+				if !r.Right.Left.IsAutoTmp() {
 					r.Right.Left = o.copyExpr(r.Right.Left, r.Right.Left.Type, false)
 				}
 

test/fixedbugs/issue43111.go

@@ -0,0 +1,70 @@
+// run
+
+// Copyright 2020 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.
+
+package main
+
+var ch chan int
+var x int
+
+func f() int {
+	close(ch)
+	ch = nil
+	return 0
+}
+
+func g() int {
+	ch = nil
+	x = 0
+	return 0
+}
+
+func main() {
+	var nilch chan int
+	var v int
+	var ok bool
+	_, _ = v, ok
+
+	ch = make(chan int)
+	select {
+	case <-ch:
+	case nilch <- f():
+	}
+
+	ch = make(chan int)
+	select {
+	case v = <-ch:
+	case nilch <- f():
+	}
+
+	ch = make(chan int)
+	select {
+	case v := <-ch: _ = v
+	case nilch <- f():
+	}
+
+	ch = make(chan int)
+	select {
+	case v, ok = <-ch:
+	case nilch <- f():
+	}
+
+	ch = make(chan int)
+	select {
+	case v, ok := <-ch: _, _ = v, ok
+	case nilch <- f():
+	}
+
+	ch1 := make(chan int, 1)
+	ch = ch1
+	x = 42
+	select {
+	case ch <- x:
+	case nilch <- g():
+	}
+	if got := <-ch1; got != 42 {
+		panic(got)
+	}
+}