go/parser: parse an ast.IndexExpr for a[]

To be consistent with Go 1.16, and to preserve as much information in
the AST as possible, parse an ast.IndexExpr with BadExpr Index for the
invalid expression a[].

A go/types test had to be adjusted to account for an additional error
resulting from this change.

We don't have a lot of test coverage for parser error recovery, so
rather than write an ad-hoc test for this issue, add a new go/types test
that checks that the indexed operand is used.

Updates #46403

Change-Id: I21e6ff4179746aaa50e530d4091fded450e69824
Reviewed-on: https://go-review.googlesource.com/c/go/+/329791
Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/parser/parser.go

@@ -1302,7 +1302,12 @@ func (p *parser) parseIndexOrSliceOrInstance(x ast.Expr) ast.Expr {
 		p.errorExpected(p.pos, "operand")
 		rbrack := p.pos
 		p.next()
-		return &ast.BadExpr{From: x.Pos(), To: rbrack}
+		return &ast.IndexExpr{
+			X:      x,
+			Lbrack: lbrack,
+			Index:  &ast.BadExpr{From: rbrack, To: rbrack},
+			Rbrack: rbrack,
+		}
 	}
 	p.exprLev++
 

src/go/types/testdata/examples/functions.go2

@@ -210,5 +210,5 @@ func _() {
 func h[] /* ERROR empty type parameter list */ ()
 
 func _() {
-	h[] /* ERROR operand */ ()
+	h /* ERROR cannot index */ [] /* ERROR operand */ ()
 }

src/go/types/testdata/fixedbugs/issue46403.src

@@ -0,0 +1,11 @@
+// Copyright 2021 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 issue46403
+
+func _() {
+	// a should be used, despite the parser error below.
+	var a []int
+	var _ = a[] // ERROR expected operand
+}