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spec: permit keys denoting embedded fields in struct literals

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For #9859.

Change-Id: I4209b41c4c09126d9ad377eea6c091c8d8a9480f
Reviewed-on: https://go-review.googlesource.com/c/go/+/734321
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
This commit is contained in:
Robert Griesemer
2026-01-06 17:17:42 -08:00
committed by Gopher Robot
parent 460522ed1d
commit 1a8f9d8141
1 changed files with 37 additions and 14 deletions
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51
doc/go_spec.html
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@@ -1,6 +1,6 @@
<!--{
"Title": "The Go Programming Language Specification",
"Subtitle": "Language version go1.26 (Jan 12, 2026)",
"Subtitle": "Language version go1.27 (March 5, 2026)",
"Path": "/ref/spec"
}-->
@@ -1118,9 +1118,7 @@ that field or method <code>f</code>.
</p>
<p>
Promoted fields act like ordinary fields
of a struct except that they cannot be used as field names in
<a href="#Composite_literals">composite literals</a> of the struct.
Promoted fields act like ordinary fields of a struct.
</p>
<p>
@@ -3175,9 +3173,9 @@ a valid composite literal type.
The types of the elements and keys must be <a href="#Assignability">assignable</a>
to the respective field, element, and key types of the LiteralType;
there is no additional conversion.
The key is interpreted as a field name for struct literals,
The key is interpreted as a field <a href="#Selectors">selector</a> for struct literals,
an index for array and slice literals, and a key for map literals.
It is an error to specify multiple elements with the same field name
It is an error to specify multiple elements with the same field selector
or constant key value.
A literal may omit the element list; such a literal evaluates
to the zero value for its type.
@@ -3212,22 +3210,28 @@ For struct literals with keys the following rules apply:
<ul>
<li>Every element must have a key.
</li>
<li>Each key must be a field name declared in the struct type.
<li>Each key must be a valid field <a href="#Selectors">selector</a>
[<a href="#Go_1.27">Go 1.27</a>] for a (possibly embedded) field
of the struct; the key selects that field.
</li>
<li>The types of the embedded fields (if any) <a href="#Selectors">traversed</a>
to reach a selected field must not be pointer types.
</li>
<li>A key must not denote a field inside an embedded struct if
that struct is also specified by another key.
</li>
<li>The element list does not need to have an element for each struct field.
Omitted fields get the zero value for that field.
</li>
<li>It is an error to specify an element for a non-exported
field of a struct belonging to a different package.
</li>
</ul>
<p>
Given the declarations
</p>
<pre>
type Point3D struct { x, y, z float64 }
type Line struct { p, q Point3D }
type Object struct { name, color string }
type Point3D struct { Object; x, y, z float64 }
type Line struct { Object; p, q Point3D }
</pre>
<p>
@@ -3235,8 +3239,18 @@ one may write
</p>
<pre>
origin := Point3D{} // zero value for Point3D
line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x
origin := Point3D{} // zero value for Point3D
line1 := Line{Object{}, origin, Point3D{y: -4, z: 12.3}} // zero value for line1.q.x
line2 := Line{name: "diagonal", q: Point3D{1, 1, 1}} // zero value for line2.Object.color, line2.p
</pre>
<p>
but field selectors may not denote overlapping fields:
</p>
<pre>
obj := Object{"edge", "black"}
line3 := Line{Object: obj, name: "diagonal"} // invalid: name denotes a field inside Object
</pre>
<h4>Array and slice literals</h4>
@@ -8787,6 +8801,15 @@ An <a href="#Alias_declarations">alias declaration</a> may declare
</li>
</ul>
<h4 id="Go_1.27">Go 1.27</h4>
<ul>
<li>
A key in a struct <a href="#Composite_literals">composite literal</a> may
be any valid field <a href="#Selectors">selector</a> for the struct type,
not just a (top-level) field name of the struct.
</li>
</ul>
<h3 id="Type_unification_rules">Type unification rules</h3>
<p>