cmd/compile/internal/syntax: better error message for index syntax error

Fixes #47704.

Change-Id: I1de9fd00baaa4b534c23f011ade54120f5153a9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/342369
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -1049,7 +1049,11 @@ loop:
 			}
 
 			// x[i:...
-			p.want(_Colon)
+			// For better error message, don't use p.want(_Colon) here (issue #47704).
+			if !p.got(_Colon) {
+				p.syntaxError("expecting : or ]")
+				p.advance(_Colon, _Rbrack)
+			}
 			p.xnest++
 			t := new(SliceExpr)
 			t.pos = pos

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

@@ -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 non-generic mode
+func _() {
+	_ = m[]   // ERROR expecting operand
+	_ = m[x,] // ERROR unexpected comma, expecting \: or \]
+	_ = 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 \: or \]
+		]
+}