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spec: add definition of "specific types" of an interface

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The notion of specific types will be used to define rules for
assignability, convertability, etc. when type parameters are
involved.

Change-Id: Ic5c134261e2a9fe05cdf25efd342f052458ab5c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/366754
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Robert Griesemer 2021-11-23 12:02:40 -08:00
parent 4da06e7b00
commit 5d8c49a5a1
1 changed files with 57 additions and 7 deletions
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64
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 Nov 22, 2021",
"Subtitle": "Version of Nov 24, 2021",
"Path": "/ref/spec"
}-->
@ -1290,8 +1290,8 @@ UnderlyingType = "~" Type .
<p>
An interface type is specified by a list of <i>interface elements</i>.
An interface element is either a method or a type element,
where a type element is a union of one or more type terms.
An interface element is either a <i>method</i> or a <i>type element</i>,
where a type element is a union of one or more <i>type terms</i>.
A type term is either a single type or a single underlying type.
</p>
@ -1926,7 +1926,60 @@ x T x is not representable by a value of T because
1e1000 float64 1e1000 overflows to IEEE +Inf after rounding
</pre>
<h3 id="Structural_interfaces">Structural interfaces</h3>
<h3 id="Structure_of_interfaces">Structure of interfaces</h3>
<p>
An interface specification which contains <a href="#Interface_types">type elements</a>
that are not interface types defines a (possibly empty) set of <i>specific types</i>.
Loosely speaking, these are the types <code>T</code> that appear in the
interface definition in terms of the form <code>T</code>, <code>~T</code>,
or in unions of such terms.
</p>
<p>
More precisely, for a given interface, the set of specific types is defined as follows:
</p>
<ul>
<li>The set of specific types of the empty interface is the empty set.
</li>
<li>The set of specific types of a non-empty interface is the intersection
of the specific types of its interface elements.
</li>
<li>The set of specific types of a method specification is the empty set.
</li>
<li>The set of specific types of a non-interface type term <code>T</code>
or <code>~T</code> is the set consisting of the type <code>T</code>.
</li>
<li>The set of specific types of a <i>union</i> of terms
<code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>
is the union of the specific types of the terms.
</li>
</ul>
<p>
If the set of specific types is empty, the interface has <i>no specific types</i>.
</p>
<p>
Examples of interfaces with their specific types:
</p>
<pre>
type Celsius float32
type Kelvin float32
interface{} // no specific types
interface{ int } // int
interface{ ~string } // string
interface{ int|~string } // int, string
interface{ Celsius|Kelvin } // Celsius, Kelvin
interface{ int; string } // no specific types (intersection is empty)
</pre>
<p>
An interface <code>T</code> is called <i>structural</i> if one of the following
@ -1966,9 +2019,6 @@ Examples of structural interfaces with their structural types:
</p>
<pre>
type Celsius float32
type Kelvin float32
interface{ int } // int
interface{ Celsius|Kelvin } // float32
interface{ ~chan int } // chan int