cmd/compile/internal/syntax: better error recovery after missing expression

Don't skip closing parentheses of any kind after a missing
expression. They are likely part of the lexical construct
enclosing the expression.

Fixes #33386.

Change-Id: Ic0abc2037ec339a345ec357ccc724b7ad2a64c00
Reviewed-on: https://go-review.googlesource.com/c/go/+/188502
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -869,7 +869,7 @@ func (p *parser) operand(keep_parens bool) Expr {
 	default:
 		x := p.bad()
 		p.syntaxError("expecting expression")
-		p.advance()
+		p.advance(_Rparen, _Rbrack, _Rbrace)
 		return x
 	}
 
@@ -1840,6 +1840,9 @@ func (p *parser) header(keyword token) (init SimpleStmt, cond Expr, post SimpleS
 		} else {
 			// asking for a '{' rather than a ';' here leads to a better error message
 			p.want(_Lbrace)
+			if p.tok != _Lbrace {
+				p.advance(_Lbrace, _Rbrace) // for better synchronization (e.g., issue #22581)
+			}
 		}
 		if keyword == _For {
 			if p.tok != _Semi {

test/fixedbugs/issue33386.go

@@ -0,0 +1,29 @@
+// errorcheck
+
+// Copyright 2019 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 that we don't get spurious follow-on errors
+// after a missing expression. Specifically, the parser
+// shouldn't skip over closing parentheses of any kind.
+
+package p
+
+func _() {
+	go func() { // no error here about goroutine
+		send <-
+	}() // ERROR "expecting expression"
+}
+
+func _() {
+	defer func() { // no error here about deferred function
+		1 +
+	}() // ERROR "expecting expression"
+}
+
+func _() {
+	_ = (1 +)             // ERROR "expecting expression"
+	_ = a[2 +]            // ERROR "expecting expression"
+	_ = []int{1, 2, 3 + } // ERROR "expecting expression"
+}