cmd/compile/internal/syntax: better syntax errors for typos in if/switch/for headers

Be more pessimistic when parsing if/switch/for headers for better error
messages when things go wrong.

Fixes #22581.

Change-Id: Ibb99925291ff53f35021bc0a59a4c9a7f695a194
Reviewed-on: https://go-review.googlesource.com/76290
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -1766,10 +1766,10 @@ func (p *parser) header(keyword token) (init SimpleStmt, cond Expr, post SimpleS
 		pos src.Pos
 		lit string // valid if pos.IsKnown()
 	}
-	if p.tok == _Semi {
+	if p.tok != _Lbrace {
 		semi.pos = p.pos()
 		semi.lit = p.lit
-		p.next()
+		p.want(_Semi)
 		if keyword == _For {
 			if p.tok != _Semi {
 				if p.tok == _Lbrace {

test/fixedbugs/issue22581.go

@@ -0,0 +1,27 @@
+// 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 p
+
+func f() {
+	if i := g()); i == j { // ERROR "unexpected \)"
+	}
+
+	if i == g()] { // ERROR "unexpected \]"
+	}
+
+	switch i := g()); i { // ERROR "unexpected \)"
+	}
+
+	switch g()] { // ERROR "unexpected \]"
+	}
+
+	for i := g()); i < y; { // ERROR "unexpected \)"
+	}
+
+	for g()] { // ERROR "unexpected \]"
+	}
+}