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<title>Issue 3142: std::foo&lt;incomplete&gt; should be ill-formed NDR</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p>
<h3 id="3142"><a href="lwg-active.html#3142">3142</a>. <code>std::foo&lt;incomplete&gt;</code> should be ill-formed NDR</h3>
<p><b>Section:</b> 16.4.5.8 <a href="https://wg21.link/res.on.functions">[res.on.functions]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2018-07-07 <b>Last modified:</b> 2018-11-27</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#res.on.functions">issues</a> in [res.on.functions].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>

<p>
16.4.5.8 <a href="https://wg21.link/res.on.functions">[res.on.functions]</a>/2 states:
<blockquote>
<p>
-2- In particular, the effects are undefined in the following cases:
</p><p>
[&hellip;]
</p><p>
(2.5) &mdash; if an incomplete type (6.9 <a href="https://wg21.link/basic.types">[basic.types]</a>) is used as a template argument
when instantiating a template component or evaluating a concept, unless specifically allowed for
that component.
</p>
</blockquote>
While undefined behavior is appropriate for the other cases specified in the earlier bullets, which
describe failure to meet (runtime) semantic requirements, "instantiating a template component or
evaluating a concept" with an incomplete type happens at compile-time, and could potentially be
diagnosed. It would therefore be more appropriate to specify that programs which do so are
ill-formed with no diagnostic required.
</p>

<p><i>[2018-11 Reflector prioritization]</i></p>

<p>Set Priority to 3</p>


<p id="res-3142"><b>Proposed resolution:</b></p>

<p>This wording is relative to <a href="https://wg21.link/n4762">N4762</a>.</p>

<p>Change 16.4.5.8 <a href="https://wg21.link/res.on.functions">[res.on.functions]</a> as indicated:</p>
<blockquote>
<p>
<del>(2.5) &mdash; if an incomplete type (6.9 <a href="https://wg21.link/basic.types">[basic.types]</a>) is used as a template
argument when instantiating a template component or evaluating a concept, unless specifically
allowed for that component.</del>
</p><p>
<ins>-?- Unless explicitly stated otherwise, a program that instantiates a template component or
evaluates a concept with an incomplete type (6.9 <a href="https://wg21.link/basic.types">[basic.types]</a>) as a template argument is
ill-formed with no diagnostic required.</ins>
</p>
</blockquote>






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