From 3c495839fe6e711b9161f8efc2d1bd474bf60916 Mon Sep 17 00:00:00 2001
From: Robert Griesemer <gri@golang.org>
Date: Thu, 16 Dec 2021 10:25:07 -0800
Subject: [PATCH] spec: describe constraint parsing ambiguity and work-around
 more precisely

The new description matches the implementation (CL 370774).

Also, in the section on type constraints, use "defines" instead of
"determines" because the constraint interface defines the type set
which is precisely the set of acceptable type arguments.

For #49482.

Change-Id: I6f30f49100e8ba8bec0a0f1b450f88cae54312eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/372874
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
---
 doc/go_spec.html | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/doc/go_spec.html b/doc/go_spec.html
index ed98f5375f..c0b224f977 100644
--- a/doc/go_spec.html
+++ b/doc/go_spec.html
@@ -2604,7 +2604,8 @@ has a corresponding (meta-)type which is called its
 <p>
 A parsing ambiguity arises when the type parameter list for a parameterized type
 declares a single type parameter with a type constraint of the form <code>*C</code>
-or <code>(C)</code>:
+or <code>(C)</code> where <code>C</code> is not a (possibly parenthesized)
+<a href="#Types">type literal</a>:
 </p>
 
 <pre>
@@ -2616,17 +2617,19 @@ type T[P (C)] …
 In these rare cases, the type parameter declaration is indistinguishable from
 the expressions <code>P*C</code> or <code>P(C)</code> and the type declaration
 is parsed as an array type declaration.
-To resolve the ambiguity, embed the constraint in an interface:
+To resolve the ambiguity, embed the constraint in an interface or use a trailing
+comma:
 </p>
 
 <pre>
 type T[P interface{*C}] …
+type T[P *C,] …
 </pre>
 
 <h4 id="Type_constraints">Type constraints</h4>
 
 <p>
-A type constraint is an <a href="#Interface_types">interface</a> that determines the
+A type constraint is an <a href="#Interface_types">interface</a> that defines the
 set of permissible type arguments for the respective type parameter and controls the
 operations supported by values of that type parameter.
 </p>