cmd/compile: fix println()

println with no arguments accidentally doesn't print a newline.

Introduced at CL 55097

Fixes #21808

Change-Id: I9fc7b4271b9b31e4c9b6078f055195dc3907b62c
Reviewed-on: https://go-review.googlesource.com/62390
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -2071,12 +2071,12 @@ func walkprint(nn *Node, init *Nodes) *Node {
 		s := nn.List.Slice()
 		t := make([]*Node, 0, len(s)*2)
 		for i, n := range s {
-			x := " "
+			t = append(t, n)
-			if len(s)-1 == i {
+			if i != len(s)-1 {
-				x = "\n"
+				t = append(t, nodstr(" "))
 			}
-			t = append(t, n, nodstr(x))
 		}
+		t = append(t, nodstr("\n"))
 		nn.List.Set(t)
 	}
 

test/fixedbugs/issue21808.go

@@ -0,0 +1,17 @@
+// run
+
+// Copyright 2017 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.
+
+// Make sure println() prints a blank line.
+
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("A")
+	println()
+	fmt.Println("B")
+}

test/fixedbugs/issue21808.out

@@ -0,0 +1,3 @@
+A
+
+B