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‎issue3396.html‎

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@@ -67,6 +67,7 @@ <h3 id="3396"><a href="lwg-defects.html#3396">3396</a>. Clarify point of referen
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<b>Submitter:</b> Jens Maurer <b>Opened:</b> 2020-02-13 <b>Last modified:</b> 2021-02-25</p>
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<p><b>Priority: </b>2
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</p>
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<p><b>View all other</b> <a href="lwg-index.html#support.srcloc.cons">issues</a> in [support.srcloc.cons].</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
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<p><b>Discussion:</b></p>
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<p><b>Addresses <a href="https://github.com/cplusplus/nbballot/issues/167">DE 169</a></b></p>

‎issue4207.html‎

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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="utf-8">
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<title>Issue 4207: Point of reference for source_location is not specified when used in an default template argument</title>
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<meta property="og:title" content="Issue 4207: Point of reference for source_location is not specified when used in an default template argument">
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<meta property="og:description" content="C++ library issue. Status: New">
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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>
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<h3 id="4207"><a href="lwg-active.html#4207">4207</a>. Point of reference for <code class='backtick'>source_location</code> is not specified when used in an default template argument</h3>
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<p><b>Section:</b> 17.8.2.2 <a href="https://wg21.link/support.srcloc.cons">[support.srcloc.cons]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Cassio Neri <b>Opened:</b> 2025-02-07 <b>Last modified:</b> 2025-02-07</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View all other</b> <a href="lwg-index.html#support.srcloc.cons">issues</a> in [support.srcloc.cons].</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>
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17.8.2.2 <a href="https://wg21.link/support.srcloc.cons">[support.srcloc.cons]</a>/2 in <a href="https://wg21.link/N5001">N5001</a> says nothing
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about using <code class='backtick'>source_location::current()</code> in an initializer of a template
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parameter. The example below suggests that gcc, clang and msvc seem to agree
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on this case. It would be nice if the Standard legitimized current practice.
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</p>
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<pre><code>
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#include &lt;source_location&gt;
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template &lt;int i = std::source_location::current().line()&gt; // line 3
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struct A {
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static constexpr int value = i;
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};
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template &lt;int i = std::source_location::current().line()&gt; // line 8
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constexpr int f() {
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return i;
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}
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static_assert(A&lt;&gt;::value == 3); // passes
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static_assert(f() == 8); // passes
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</code></pre>
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<p><i>[2025-02-07; Jonathan provides wording]</i></p>
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<p>
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For a default argument of a function parameter, <code class='backtick'>current()</code> is recommended
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to return the location of the caller that makes use of that default argument.
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For a default template argument, the location would be determined by the
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template's point of instantiation (13.8.4.1 <a href="https://wg21.link/temp.point">[temp.point]</a>) which would
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not always do what users expect. Using the location of the default template
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argument in the template declaration seems sensible and predictable,
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and matches existing practice. Arguably, this doesn't need to be stated
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because it's just "exactly where the <code class='backtick'>current()</code> call appear in the source".
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The other cases in the Remarks paragraph are situations where the preferred
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location is different, because we want to know where it's used, not defined.
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</p>
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<p id="res-4207"><b>Proposed resolution:</b></p>
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<p>
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This wording is relative to <a href="https://wg21.link/N5001">N5001</a>.
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</p>
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<ol>
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<li>
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Modify 17.8.2.2 <a href="https://wg21.link/support.srcloc.cons">[support.srcloc.cons]</a> as indicated:
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<blockquote>
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-2- <i>Remarks</i>:
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Any call to <code class='backtick'>current</code> that appears as a default member initializer
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(11.4 <a href="https://wg21.link/class.mem">[class.mem]</a>), or as a subexpression thereof,
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should correspond to the location of the constructor definition
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or aggregate initialization that uses the default member initializer.
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Any call to <code class='backtick'>current</code> that appears as a default argument
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(9.3.4.7 <a href="https://wg21.link/dcl.fct.default">[dcl.fct.default]</a>), or as a subexpression thereof,
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should correspond to the location of the invocation of the function that
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uses the default argument (7.6.1.3 <a href="https://wg21.link/expr.call">[expr.call]</a>).
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Any call to <code class='backtick'>current</code> that appears as a default template argument
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(13.2 <a href="https://wg21.link/temp.param">[temp.param]</a>), or as a subexpression thereof,
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should correspond to the location where the default template argument
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is specified.
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</blockquote>
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</li>
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</ol>
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</body>
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</html>

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