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spec: adjust rules for specific types once more

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Introduce a (local) notion of a set of representative types,
which serves as a representation/approximation of an
interface's actual type set. If the set of representative
types is is non-empty and finite, it corresponds to the set
of specific types of the interface.

In the implementation, the set of representative types serves
as a finite representation of an interface's type set, together
with the set of methods.

Change-Id: Ib4c6cd5e17b81197672e4247be9737dd2cb6b56f
Reviewed-on: https://go-review.googlesource.com/c/go/+/376834
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Robert Griesemer 2022-01-07 18:20:22 -08:00
parent 9bce08999a
commit 3df4472201
1 changed files with 17 additions and 11 deletions
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28
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 6, 2022",
"Subtitle": "Version of Jan 10, 2022",
"Path": "/ref/spec"
}-->
@ -1980,30 +1980,34 @@ or in unions of such terms.
</p>
<p>
More precisely, for a given interface, the set 𝑆 of specific types is defined as follows:
More precisely, for a given interface, the set of specific types corresponds to
the set 𝑅 of representative types of the interface, if 𝑅 is non-empty and finite.
Otherwise, if 𝑅 is empty or infinite, the interface has <i>no specific types</i>.
</p>
<p>
For a given interface, type element or type term, the set 𝑅 of representative types is defined as follows:
</p>
<ul>
<li>For an interface with no type elements, 𝑆 is the empty set.
<li>For an interface with no type elements, 𝑅 is the (infinite) set of all types.
</li>
<li>For an interface with type elements, 𝑆 is the intersection
of the specific types of its type elements with specific types
(type elements that have no specific types are ignored).
<li>For an interface with type elements,
𝑅 is the intersection of the representative types of its type elements.
</li>
<li>For a non-interface type term <code>T</code>
or <code>~T</code>, 𝑆 is the set consisting of the type <code>T</code>.
<li>For a non-interface type term <code>T</code> or a term of the form <code>~T</code>,
𝑅 is the set consisting of the type <code>T</code>.
</li>
<li>For a <i>union</i> of terms
<code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>,
𝑆 is the union of the specific types of the terms.
𝑅 is the union of the representative types of the terms.
</li>
</ul>
<p>
If 𝑆 is empty, the interface has <i>no specific types</i>.
An interface may have specific types even if its <a href="#Interface_types">type set</a>
is empty.
</p>
@ -2021,8 +2025,10 @@ interface{ int } // int
interface{ ~string } // string
interface{ int|~string } // int, string
interface{ Celsius|Kelvin } // Celsius, Kelvin
interface{ float64|any } // no specific types (union is all types)
interface{ int; m() } // int (but type set is empty because int has no method m)
interface{ int; any } // int (any has no specific types and is ignored)
interface{ ~int; m() } // int (but type set is infinite because many integer types have a method m)
interface{ int; any } // int
interface{ int; string } // no specific types (intersection is empty)
</pre>