cmd/compile: don't permit declarations in post statement of for loop

Report syntax error that was missed when moving to new parser.

Fixes #19610.

Change-Id: Ie5625f907a84089dc56fcccfd4f24df546042783
Reviewed-on: https://go-review.googlesource.com/38375Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -1722,6 +1722,9 @@ func (p *parser) header(keyword token) (init SimpleStmt, cond Expr, post SimpleS
 			p.want(_Semi)
 			if p.tok != _Lbrace {
 				post = p.simpleStmt(nil, false)
+				if a, _ := post.(*AssignStmt); a != nil && a.Op == Def {
+					p.syntax_error_at(a.Pos(), "cannot declare in post statement of for loop")
+				}
 			}
 		} else if p.tok != _Lbrace {
 			condStmt = p.simpleStmt(nil, false)

test/fixedbugs/issue19610.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() {
+	for ; ; x := 1 { // ERROR "cannot declare in post statement"
+		_ = x
+		break
+	}
+}