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spec: add section on comparable constraint
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>
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<!--{
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"Title": "The Go Programming Language Specification - Go 1.18 Draft (incomplete)",
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"Subtitle": "Version of Jan 28, 2022",
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"Subtitle": "Version of Jan 31, 2022",
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"Path": "/ref/spec"
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}-->
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@ -2656,6 +2656,53 @@ may be omitted for convenience:
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type Constraint ~int // illegal: ~int is not inside a type parameter list
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</pre>
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<!--
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We should be able to simplify the rules for comparable or delegate some of them
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elsewhere once we have a section that clearly defines how interfaces implement
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other interfaces based on their type sets. But this should get us going for now.
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-->
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<p>
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The <a href="#Predeclared_identifiers">predeclared</a>
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<a href="#Interface_types">interface type</a> <code>comparable</code>
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denotes the set of all concrete (non-interface) types that are
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<a href="#Comparison_operators">comparable</a>. Specifically,
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a type <code>T</code> implements <code>comparable</code> if:
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</p>
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<ul>
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<li>
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<code>T</code> is not an interface type and <code>T</code> supports the operations
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<code>==</code> and <code>!=</code>; or
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</li>
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<li>
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<code>T</code> is an interface type and each type in <code>T</code>'s
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<a href="#Interface_types">type set</a> implements <code>comparable</code>.
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</li>
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</ul>
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<p>
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Even though interfaces that are not type parameters can be
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<a href="#Comparison_operators">compared</a>
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(possibly causing a run-time panic) they do not implement
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<code>comparable</code>.
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</p>
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<pre>
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int // implements comparable
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[]byte // does not implement comparable (slices cannot be compared)
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interface{} // does not implement comparable (see above)
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interface{ ~int | ~string } // type parameter only: implements comparable
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interface{ comparable } // type parameter only: implements comparable
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interface{ ~int | ~[]byte } // type parameter only: does not implement comparable (not all types in the type set are comparable)
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</pre>
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<p>
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The <code>comparable</code> interface and interfaces that (directly or indirectly) embed
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<code>comparable</code> may only be used as type constraints. They cannot be the types of
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values or variables, or components of other, non-interface types.
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</p>
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<h3 id="Variable_declarations">Variable declarations</h3>
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<p>
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