From 753ad5e1d59db9073abba07627723fa3267a52fd Mon Sep 17 00:00:00 2001
From: Robert Griesemer <gri@golang.org>
Date: Mon, 21 Nov 2022 16:02:20 -0800
Subject: [PATCH] spec: document that trailing comma is valid after index in
 index expressions

At parse time we don't know if a[i] is an index expression or a
type (or function) instantiation. Because instantiations accept
a list of type arguments, and argument lists permit a trailing
comma, a[i,] is either an instantiation or index expression.

Document that a trailing comma is permitted in the syntax for
index expressions.

For comparison, the same problem arises with conversions which
cannot be distinguished from function calls at parse time. The
spec also permits a trailing comma for conversions T(x,). The
grammar adjustment is the same (see line 5239).

Fixes #55007.

Change-Id: Ib9101efe52031589eb95a428cc6dff940d939f9e
Reviewed-on: https://go-review.googlesource.com/c/go/+/452618
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
---
 doc/go_spec.html | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/doc/go_spec.html b/doc/go_spec.html
index 3dcab9c4a6..630012ded8 100644
--- a/doc/go_spec.html
+++ b/doc/go_spec.html
@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of November 10, 2022",
+	"Subtitle": "Version of November 21, 2022",
 	"Path": "/ref/spec"
 }-->
 
@@ -3305,7 +3305,7 @@ PrimaryExpr =
 	PrimaryExpr Arguments .
 
 Selector       = "." identifier .
-Index          = "[" Expression "]" .
+Index          = "[" Expression [ "," ] "]" .
 Slice          = "[" [ Expression ] ":" [ Expression ] "]" |
                  "[" [ Expression ] ":" Expression ":" Expression "]" .
 TypeAssertion  = "." "(" Type ")" .
-- 
2.50.0