New issue from Cassio Neri: Point of reference for source_location is not specified when used in an default template argument

Author: jwakely

xml/issue4207.xml

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+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4207" status="New">
+<title>Point of reference for `source_location` is not specified when used in an default template argument</title>
+<section><sref ref="[support.srcloc.cons]"/></section>
+<submitter>Cassio Neri</submitter>
+<date>07 Feb 2025</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+<sref ref="[support.srcloc.cons]"/>/2 in <paper num="N5001"/> says nothing
+about using `source_location::current()` in an initializer of a template
+parameter. The example below suggests that gcc, clang and msvc seem to agree
+on this case. It would be nice if the Standard legitimized current practice.
+</p>
+<pre><code>
+#include &lt;source_location&gt;
+
+template &lt;int i = std::source_location::current().line()&gt; // line 3
+struct A {
+  static constexpr int value = i;
+};
+
+template &lt;int i = std::source_location::current().line()&gt; // line 8
+constexpr int f() {
+  return i;
+}
+
+static_assert(A&lt;&gt;::value == 3); // passes
+static_assert(f() == 8); // passes
+</code></pre>
+
+<note>2025-02-07; Jonathan provides wording</note>
+<p>
+For a default argument of a function parameter, `current()` is recommended
+to return the location of the caller that makes use of that default argument.
+For a default template argument, the location would be determined by the
+template's point of instantiation (<sref ref="[temp.point]"/>) which would
+not always do what users expect. Using the location of the default template
+argument in the template declaration seems sensible and predictable,
+and matches existing practice. Arguably, this doesn't need to be stated
+because it's just "exactly where the `current()` call appear in the source".
+The other cases in the Remarks paragraph are situations where the preferred
+location is different, because we want to know where it's used, not defined.
+</p>
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to <paper num="N5001"/>.
+</p>
+
+<ol>
+<li>
+Modify <sref ref="[support.srcloc.cons]"/> as indicated:
+<blockquote>
+-2- <i>Remarks</i>:
+Any call to `current` that appears as a default member initializer
+(<sref ref="[class.mem]"/>), or as a subexpression thereof,
+should correspond to the location of the constructor definition
+or aggregate initialization that uses the default member initializer.
+Any call to `current` that appears as a default argument
+(<sref ref="[dcl.fct.default]"/>), or as a subexpression thereof,
+should correspond to the location of the invocation of the function that
+uses the default argument (<sref ref="[expr.call]"/>).
+Any call to `current` that appears as a default template argument
+(<sref ref="[temp.param]"/>), or as a subexpression thereof,
+should correspond to the location where the default template argument
+is specified.
+</blockquote>
+</li>
+</ol>
+</resolution>
+
+</issue>