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spec: adjust assignability rules for type parameters 

Change-Id: I3c4d8bdb5e92ee7fdca9593fb043f94f467755e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/365434
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Robert Griesemer 2021-11-18 16:11:10 -08:00
parent 003e7faf53
commit 939480033a
1 changed files with 33 additions and 9 deletions
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@ -1848,21 +1848,22 @@ A value <code>x</code> is <i>assignable</i> to a <a href="#Variables">variable</
</li>
<li>
<code>x</code>'s type <code>V</code> and <code>T</code> have identical
<a href="#Types">underlying types</a> and at least one of <code>V</code>
or <code>T</code> is not a <a href="#Type_definitions">defined</a> type.
<a href="#Underlying_types">underlying types</a> and at least one of <code>V</code>
or <code>T</code> is not a <a href="#Types">named type</a>.
</li>
<li>
<code>T</code> is an interface type and
<code>x</code>'s type <code>V</code> and <code>T</code> are channel types with
identical element types, <code>V</code> is a bidirectional channel,
and at least one of <code>V</code> or <code>T</code> is not a <a href="#Types">named type</a>.
</li>
<li>
<code>T</code> is an interface type, but not a type parameter, and
<code>x</code> <a href="#Interface_types">implements</a> <code>T</code>.
</li>
<li>
<code>x</code> is a bidirectional channel value, <code>T</code> is a channel type,
<code>x</code>'s type <code>V</code> and <code>T</code> have identical element types,
and at least one of <code>V</code> or <code>T</code> is not a defined type.
</li>
<li>
<code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code>
is a pointer, function, slice, map, channel, or interface type.
is a pointer, function, slice, map, channel, or interface type,
but not a type parameter.
</li>
<li>
<code>x</code> is an untyped <a href="#Constants">constant</a>
@ -1871,6 +1872,29 @@ by a value of type <code>T</code>.
</li>
</ul>
<p>
Additionally, if <code>x's</code> type <code>V</code> or <code>T</code> are type parameters
with <a href="#Structure_of_interfaces">specific types</a>, <code>x</code>
is assignable to a variable of type <code>T</code> if one of the following conditions applies:
</p>
<ul>
<li>
<code>x</code> is the predeclared identifier <code>nil</code>, <code>T</code> is
a type parameter, and <code>x</code> is assignable to each specific type of
<code>T</code>.
</li>
<li>
<code>V</code> is not a <a href="#Types">named type</a>, <code>T</code> is
a type parameter, and <code>x</code> is assignable to each specific type of
<code>T</code>.
</li>
<li>
<code>V</code> is a type parameter and <code>T</code> is not a named type,
and values of each specific type of <code>V</code> are assignable
to <code>T</code>.
</li>
</ul>
<h3 id="Representability">Representability</h3>