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spec: add section on comparable constraint

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For #50646.
Fixes #50791.

Change-Id: I8fec25ae3f0280c5b5a778011d23842b886ba79e
Reviewed-on: https://go-review.googlesource.com/c/go/+/381896
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Robert Griesemer 2022-01-28 15:34:40 -08:00
parent 4fea5935f5
commit 9152e21132
1 changed files with 48 additions and 1 deletions
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49
doc/go_spec.html
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@ -1,6 +1,6 @@
<!--{
"Title": "The Go Programming Language Specification - Go 1.18 Draft (incomplete)",
"Subtitle": "Version of Jan 28, 2022",
"Subtitle": "Version of Jan 31, 2022",
"Path": "/ref/spec"
}-->
@ -2656,6 +2656,53 @@ may be omitted for convenience:
type Constraint ~int // illegal: ~int is not inside a type parameter list
</pre>
<!--
We should be able to simplify the rules for comparable or delegate some of them
elsewhere once we have a section that clearly defines how interfaces implement
other interfaces based on their type sets. But this should get us going for now.
-->
<p>
The <a href="#Predeclared_identifiers">predeclared</a>
<a href="#Interface_types">interface type</a> <code>comparable</code>
denotes the set of all concrete (non-interface) types that are
<a href="#Comparison_operators">comparable</a>. Specifically,
a type <code>T</code> implements <code>comparable</code> if:
</p>
<ul>
<li>
<code>T</code> is not an interface type and <code>T</code> supports the operations
<code>==</code> and <code>!=</code>; or
</li>
<li>
<code>T</code> is an interface type and each type in <code>T</code>'s
<a href="#Interface_types">type set</a> implements <code>comparable</code>.
</li>
</ul>
<p>
Even though interfaces that are not type parameters can be
<a href="#Comparison_operators">compared</a>
(possibly causing a run-time panic) they do not implement
<code>comparable</code>.
</p>
<pre>
int // implements comparable
[]byte // does not implement comparable (slices cannot be compared)
interface{} // does not implement comparable (see above)
interface{ ~int | ~string } // type parameter only: implements comparable
interface{ comparable } // type parameter only: implements comparable
interface{ ~int | ~[]byte } // type parameter only: does not implement comparable (not all types in the type set are comparable)
</pre>
<p>
The <code>comparable</code> interface and interfaces that (directly or indirectly) embed
<code>comparable</code> may only be used as type constraints. They cannot be the types of
values or variables, or components of other, non-interface types.
</p>
<h3 id="Variable_declarations">Variable declarations</h3>
<p>