cmd/compile/internal/parser: improved syntax error for incorrect if/for/switch header

Starting the error message with "expecting" rather than "missing"
causes the syntax error mechanism to add additional helpful info
(it recognizes "expecting" but not "missing").

Fixes #17328.

Change-Id: I8482ca5e5a6a6b22e0ed0d831b7328e264156334
Reviewed-on: https://go-review.googlesource.com/36637
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/syntax/parser.go

@@ -1690,7 +1690,7 @@ func (p *parser) stmtBody(context string) []Stmt {
 	}
 
 	if !p.got(_Lbrace) {
-		p.syntax_error("missing { after " + context)
+		p.syntax_error("expecting { after " + context)
 		p.advance(_Name, _Rbrace)
 	}
 

test/fixedbugs/issue17328.go

+// errorcheck
+
+// 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.
+
+package main
+
+func main() {
+	i := 0
+	for ; ; i++) { // ERROR "unexpected \), expecting { after for clause"
+	}
+}

test/syntax/semi1.go

@@ -7,7 +7,7 @@
 package main
 
 func main() {
-	if x; y		// ERROR "missing .*{.* after if clause|undefined"
+	if x; y		// ERROR "expected .*{.* after if clause|undefined"
 	{
 		z	// GCCGO_ERROR "undefined"
 

test/syntax/semi3.go

@@ -7,7 +7,7 @@
 package main
 
 func main() {
-	for x; y; z	// ERROR "missing .*{.* after for clause|undefined"
+	for x; y; z	// ERROR "expected .*{.* after for clause|undefined"
 	{
 		z	// GCCGO_ERROR "undefined"
 

test/syntax/semi4.go

@@ -13,5 +13,5 @@ package main
 
 func main() {
 	for x		// GCCGO_ERROR "undefined"
-	{		// ERROR "missing .*{.* after for clause|missing operand"
-		z	// ERROR "undefined|missing { after for clause"
+	{		// ERROR "expecting .*{.* after for clause|missing operand"
+		z	// ERROR "undefined|expecting { after for clause"