cmd/compile/internal/types2: better error message for index syntax error (follow-up)

For #47704.

Change-Id: I09e6f638df0cd456a20a3b68ab55c47bb5b1f555
Reviewed-on: https://go-review.googlesource.com/c/go/+/342370
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

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

@@ -1049,10 +1049,15 @@ loop:
 			}
 
 			// x[i:...
-			// For better error message, don't use p.want(_Colon) here (issue #47704).
+			// For better error message, don't simply use p.want(_Colon) here (issue #47704).
 			if !p.got(_Colon) {
-				p.syntaxError("expecting : or ]")
+				if p.mode&AllowGenerics == 0 {
-				p.advance(_Colon, _Rbrack)
+					p.syntaxError("expecting : or ]")
+					p.advance(_Colon, _Rbrack)
+				} else {
+					p.syntaxError("expecting comma, : or ]")
+					p.advance(_Comma, _Colon, _Rbrack)
+				}
 			}
 			p.xnest++
 			t := new(SliceExpr)

src/cmd/compile/internal/syntax/testdata/issue47704.go2

@@ -0,0 +1,18 @@
+// 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 p
+
+// error messages for parser in generic mode
+func _() {
+	_ = m[] // ERROR expecting operand
+	_ = m[x,]
+	_ = m[x /* ERROR unexpected a */ a b c d]
+}
+
+// test case from the issue
+func f(m map[int]int) int {
+	return m[0 // ERROR expecting comma, \: or \]
+		]
+}