cmd/compile/internal/syntax: better error message for incorrect if/switch header

Fixes #23664.

Change-Id: Ic0637e9f896b2fc6502dfbab2d1c4de3c62c0bd2
Reviewed-on: https://go-review.googlesource.com/104616
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>

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

@@ -1824,7 +1824,8 @@ func (p *parser) header(keyword token) (init SimpleStmt, cond Expr, post SimpleS
 			semi.lit = p.lit
 			p.next()
 		} else {
-			p.want(_Semi)
+			// asking for a '{' rather than a ';' here leads to a better error message
+			p.want(_Lbrace)
 		}
 		if keyword == _For {
 			if p.tok != _Semi {

test/fixedbugs/issue23664.go

@@ -0,0 +1,17 @@
+// errorcheck
+
+// Copyright 2018 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.
+
+// Verify error messages for incorrect if/switch headers.
+
+package p
+
+func f() {
+	if f() true { // ERROR "unexpected true, expecting {"
+	}
+	
+	switch f() true { // ERROR "unexpected true, expecting {"
+	}
+}