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<h1 class="title" style="text-align:center">Expose <code class="sourceCode cpp">std<span class="op">::</span><em>basic-format-string</em><span class="op"><</span>charT, Args<span class="op">...></span></code></h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2508R2</td>
</tr>
<tr>
<td>Date:</td>
<td>2022-02-04</td>
</tr>
<tr>
<td style="vertical-align:top">Project:</td>
<td>Programming Language C++</td>
</tr>
<tr>
<td style="vertical-align:top">Audience:</td>
<td>
LEWG<br>
</td>
</tr>
<tr>
<td style="vertical-align:top">Reply-to:</td>
<td>
Barry Revzin<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
</td>
</tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#revision-history"><span class="toc-section-number">1</span> Revision History<span></span></a></li>
<li><a href="#introduction"><span class="toc-section-number">2</span> Introduction<span></span></a>
<ul>
<li><a href="#why-exposition-only"><span class="toc-section-number">2.1</span> Why exposition-only?<span></span></a></li>
<li><a href="#yet-another-dr-against-c20-are-you-serious"><span class="toc-section-number">2.2</span> Yet another DR against C++20, are you serious?<span></span></a></li>
<li><a href="#also-need-a-getter"><span class="toc-section-number">2.3</span> Also need a getter<span></span></a></li>
<li><a href="#feature-test-macro"><span class="toc-section-number">2.4</span> Feature-test macro?<span></span></a></li>
</ul></li>
<li><a href="#wording"><span class="toc-section-number">3</span> Wording<span></span></a>
<ul>
<li><a href="#feature-test-macro-1"><span class="toc-section-number">3.1</span> Feature-test macro<span></span></a></li>
</ul></li>
<li><a href="#bibliography"><span class="toc-section-number">4</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history"><span class="header-section-number">1</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<p>Since <span class="citation" data-cites="P2508R1">[<a href="#ref-P2508R1" role="doc-biblioref">P2508R1</a>]</span>: wording update.</p>
<p>Since <span class="citation" data-cites="P2508R0">[<a href="#ref-P2508R0" role="doc-biblioref">P2508R0</a>]</span>: making <code class="sourceCode cpp"><em>basic-format-string</em></code> non-exposition only isn’t sufficient, also actually need a way to pull out the underlying format string if we want to eventually invoke <code class="sourceCode cpp">vformat</code>.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="introduction"><span class="header-section-number">2</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p><span class="citation" data-cites="P2216R3">[<a href="#ref-P2216R3" role="doc-biblioref">P2216R3</a>]</span>, in order to improve type safety around the <code class="sourceCode cpp">std<span class="op">::</span>format</code> API, introduced the type <code class="sourceCode cpp">std<span class="op">::</span><em>basic-format-string</em><span class="op"><</span>charT, Args<span class="op">...></span></code> as a way to check to make sure that the format string actually matches the arguments provided. The example used in that paper is:</p>
<blockquote>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a>std<span class="op">::</span>string s <span class="op">=</span> std<span class="op">::</span>format<span class="op">(</span><span class="st">"{:d}"</span>, <span class="st">"I am not a number"</span><span class="op">)</span>;</span></code></pre></div>
</blockquote>
<p>Originally, this would throw an exception at runtime. With the change, this now is a compile error. Which is a very nice improvement - catching bugs at compile time is awesome.</p>
<p>Unfortunately, the ability to validate format strings at compile time is currently limited to specifically those functions provided by the standard library. In particular, <code class="sourceCode cpp">std<span class="op">::</span>format</code>, <code class="sourceCode cpp">std<span class="op">::</span>format_to</code>, <code class="sourceCode cpp">std<span class="op">::</span>format_to_n</code>, and <code class="sourceCode cpp">std<span class="op">::</span>formatted_size</code>. But users cannot take advantage of this in their own code.</p>
<p>As Joseph Thompson pointed out in <span class="citation" data-cites="std-discussion">[<a href="#ref-std-discussion" role="doc-biblioref">std-discussion</a>]</span>:</p>
<blockquote>
<p>However, consider the common use case:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Args<span class="op">></span></span>
<span id="cb2-2"><a href="#cb2-2"></a><span class="dt">void</span> log<span class="op">(</span>std<span class="op">::</span>format_string<span class="op"><</span>Args<span class="op">...></span> s, Args<span class="op">&&...</span> args<span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-3"><a href="#cb2-3"></a> <span class="cf">if</span> <span class="op">(</span>logging_enabled<span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-4"><a href="#cb2-4"></a> log_raw<span class="op">(</span>std<span class="op">::</span>format<span class="op">(</span>s, std<span class="op">::</span>forward<span class="op"><</span>Args<span class="op">>(</span>args<span class="op">)...))</span>;</span>
<span id="cb2-5"><a href="#cb2-5"></a> <span class="op">}</span></span>
<span id="cb2-6"><a href="#cb2-6"></a><span class="op">}</span></span></code></pre></div>
<p>Without a specified <code class="sourceCode cpp">std<span class="op">::</span>format_string</code> type, is it possible to forward arguments to <code class="sourceCode cpp">std<span class="op">::</span>format</code> with compile-time format string checks? I’m thinking about this as an average user of the standard library, so I’m not looking for workarounds. Is this a defect?</p>
</blockquote>
<p>Today, if users want to statically verify their format strings… what can they do? Originally, I thought that you could write a macro to wrap the call to <code class="sourceCode cpp">log</code>, so that you can attempt to pass the expressions as they are to try to statically verify them. But this compiles just fine:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="kw">using</span> T <span class="op">=</span> <span class="kw">decltype</span><span class="op">(</span>std<span class="op">::</span>format<span class="op">(</span><span class="st">"{:d}"</span>, <span class="st">"I am not a number"</span><span class="op">))</span>; <span class="co">// ok</span></span>
<span id="cb3-2"><a href="#cb3-2"></a><span class="kw">static_assert</span><span class="op">(</span><span class="kw">requires</span> <span class="op">{</span> std<span class="op">::</span>format<span class="op">(</span><span class="st">"{:d}"</span>, <span class="st">"I am not a number"</span><span class="op">)</span>; <span class="op">})</span>; <span class="co">// ok</span></span></code></pre></div>
<p>Because… of course it compiles. The way that this check works in format today is that <code class="sourceCode cpp"><em>basic-format-string</em><span class="op"><</span>charT, Args<span class="op">...></span></code> has a <code class="sourceCode cpp"><span class="kw">consteval</span></code> constructor that just isn’t a constant-expression if the value of the format string doesn’t match the arguments.</p>
<p>But constant-expression-ness (or, in general, values) don’t affect overload resolution, so there’s no way to check this externally (there were some papers that considered value-based constraints which would’ve let this work <span class="citation" data-cites="P1733R0">[<a href="#ref-P1733R0" role="doc-biblioref">P1733R0</a>]</span> <span class="citation" data-cites="P2049R0">[<a href="#ref-P2049R0" role="doc-biblioref">P2049R0</a>]</span>, which aren’t without potential issue <span class="citation" data-cites="P2089R0">[<a href="#ref-P2089R0" role="doc-biblioref">P2089R0</a>]</span>, but we don’t have something like that today).</p>
<p>The only way for Joseph (or you or me or …) to get this behavior in <code class="sourceCode cpp">log</code> is to basically manually re-implement <code class="sourceCode cpp"><em>basic-format-string</em></code>. That seems like an unreasonable burden to place for such a useful facility. The standard library already has to provide it, why not provide it under a guaranteed name that we can use?</p>
<h2 data-number="2.1" id="why-exposition-only"><span class="header-section-number">2.1</span> Why exposition-only?<a href="#why-exposition-only" class="self-link"></a></h2>
<p>There are two reasons why <code class="sourceCode cpp"><em>basic-format-string</em><span class="op"><</span>charT, Args<span class="op">...></span></code> (and its more specialized alias templates, <code class="sourceCode cpp"><em>format-string</em><span class="op"><</span>Args<span class="op">...></span></code> and <code class="sourceCode cpp"><em>wformat-string</em><span class="op"><</span>Args<span class="op">...></span></code>) are exposition-only.</p>
<p>The first is that, as a late DR against C++20, <span class="citation" data-cites="P2216R3">[<a href="#ref-P2216R3" role="doc-biblioref">P2216R3</a>]</span> was trying to limit its scope. Totally understandable.</p>
<p>The second is that it is possible with future language features to be able to do this better. For example, if we had <code class="sourceCode cpp"><span class="kw">constexpr</span></code> function parameters <span class="citation" data-cites="P1045R1">[<a href="#ref-P1045R1" role="doc-biblioref">P1045R1</a>]</span> or some kind of hygienic macro facility <span class="citation" data-cites="P1221R1">[<a href="#ref-P1221R1" role="doc-biblioref">P1221R1</a>]</span>, then the whole shape of this API would be different. For <code class="sourceCode cpp"><span class="kw">constexpr</span></code> function parameters, for instance, the first argument to <code class="sourceCode cpp">std<span class="op">::</span>format</code> would probably be a <code class="sourceCode cpp"><span class="kw">constexpr</span> std<span class="op">::</span>string</code> (or some such) and then there would just be a <code class="sourceCode cpp"><span class="kw">consteval</span></code> function you could call like <code class="sourceCode cpp">validate_format_string<span class="op"><</span>Args<span class="op">...>(</span>str<span class="op">)</span></code>. In which case, we could expose <code class="sourceCode cpp">validate_format_string</code> and the implementation of <code class="sourceCode cpp">log</code> could call that directly too. And that would be pretty nice!</p>
<p>But we won’t have <code class="sourceCode cpp"><span class="kw">constexpr</span></code> function parameters or a hygienic macros in C++23, so the solution that we have to this problem today is <code class="sourceCode cpp"><em>basic-format-string</em><span class="op"><</span>charT, Args<span class="op">...></span></code>. Since we have this solution, and it works, we shouldn’t just restrict it to be used by the standard library internally. If eventually we adopt a better way of doing this checking, we can expose that too. Better give users a useful facility today that’s possibly worse than a facility we might be able to provide in C++26, rather than having them have to manually write their own version (which is certainly doable, though tedious and error-prone).</p>
<h2 data-number="2.2" id="yet-another-dr-against-c20-are-you-serious"><span class="header-section-number">2.2</span> Yet another DR against C++20, are you serious?<a href="#yet-another-dr-against-c20-are-you-serious" class="self-link"></a></h2>
<p>This doesn’t strictly have to be a DR, and could certainly just be a C++23 feature. Although it would be nice to have sooner rather than later.</p>
<p>Even if an implementation already ships with a <code class="sourceCode cpp">std<span class="op">::</span>_Ugly<span class="op">::</span>_Basic_Format__Construct__Additional__Underscores<span class="op"><</span>charT, Args<span class="op">...></span></code> (which Microsoft is <a href="https://github.com/microsoft/STL/pull/2221/">close to doing</a>, with a slightly more reasonable ugly name), they can still provide the new names (<code class="sourceCode cpp">std<span class="op">::</span>basic_format_string</code>, <code class="sourceCode cpp">std<span class="op">::</span>format_string</code>, and <code class="sourceCode cpp">std<span class="op">::</span>wformat_string</code>) without having to worry about ABI, since these are templates. Or we could ask Charlie very nicely to simply rename <code class="sourceCode cpp">_Basic_format_string</code> to <code class="sourceCode cpp">basic_format_string</code> as that PR is going out the door.</p>
<p>Either way, it would be nice to avoid specifying <code class="sourceCode cpp">std<span class="op">::</span>basic_format_string</code> as an alias to <code class="sourceCode cpp">std<span class="op">::</span><em>basic-format-string</em></code> (which, technically, is an option, and obviously has no ABI impact or take much implementation effort at all). That would be weird, but probably not even particularly notably weird. Note that specifying <code class="sourceCode cpp">std<span class="op">::</span>basic_format_string</code> as a class template does prohibit implementing it as an alias template to <code class="sourceCode cpp">std<span class="op">::</span>_Basic_format_string</code>, as this difference is observable.</p>
<p>I leave it up to the discretion of LEWG, Charlie, and the ghost of C++20 Past whether or not we ever actually want to declare C++20 complete.</p>
<h2 data-number="2.3" id="also-need-a-getter"><span class="header-section-number">2.3</span> Also need a getter<a href="#also-need-a-getter" class="self-link"></a></h2>
<p>Making <code class="sourceCode cpp"><em>basic-format-string</em></code> publicly accessible is enough to make the original example work, since <code class="sourceCode cpp">std<span class="op">::</span>format</code> just takes a <code class="sourceCode cpp">std<span class="op">::</span>format_string<span class="op"><</span>Args<span class="op">...></span></code>, so if we construct one in advance and pass it in, that’s sufficient:</p>
<blockquote>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Args<span class="op">></span></span>
<span id="cb4-2"><a href="#cb4-2"></a><span class="dt">void</span> log<span class="op">(</span>std<span class="op">::</span>format_string<span class="op"><</span>Args<span class="op">...></span> s, Args<span class="op">&&...</span> args<span class="op">)</span> <span class="op">{</span></span>
<span id="cb4-3"><a href="#cb4-3"></a> <span class="cf">if</span> <span class="op">(</span>logging_enabled<span class="op">)</span> <span class="op">{</span></span>
<span id="cb4-4"><a href="#cb4-4"></a> log_raw<span class="op">(</span>std<span class="op">::</span>format<span class="op">(</span>s, std<span class="op">::</span>forward<span class="op"><</span>Args<span class="op">>(</span>args<span class="op">)...))</span>;</span>
<span id="cb4-5"><a href="#cb4-5"></a> <span class="op">}</span></span>
<span id="cb4-6"><a href="#cb4-6"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>However, it’s not enough for us with other APIs. We may want to do static validation in the front-end of our logger, but then ship everything over to the back-end to do actual logging, in a way that could support shipping the actual format string but not its type. This is still useful, as it lets us get the benefit of doing checks up front. It’s just that ultimately, instead of calling <code class="sourceCode cpp">std<span class="op">::</span>format</code> to do the actual formatting, we call <code class="sourceCode cpp">std<span class="op">::</span>vformat</code> (which just takes a <code class="sourceCode cpp">string_view</code> and a <code class="sourceCode cpp">format_args</code>).</p>
<p>In order to do so, we need some way to pull out the underlying <code class="sourceCode cpp">string_view</code> from a <code class="sourceCode cpp">std<span class="op">::</span>format<span class="op"><</span>Args<span class="op">...></span></code>. A simple getter suffices, but we do need that getter. So this paper isn’t simply making names non-exposition-only, it’s also adding a function.</p>
<h2 data-number="2.4" id="feature-test-macro"><span class="header-section-number">2.4</span> Feature-test macro?<a href="#feature-test-macro" class="self-link"></a></h2>
<p>It is definitely important to provide a feature-test macro for this, which will allow <code class="sourceCode cpp">log</code> above to conditionally opt in to this feature if possible:</p>
<blockquote>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a><span class="pp">#if __cpp_lib_format >= whatever</span></span>
<span id="cb5-2"><a href="#cb5-2"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Args<span class="op">></span></span>
<span id="cb5-3"><a href="#cb5-3"></a> <span class="kw">using</span> my_format_string <span class="op">=</span> std<span class="op">::</span>format_string<span class="op"><</span>std<span class="op">::</span>type_identity_t<span class="op"><</span>Args<span class="op">>...></span>;</span>
<span id="cb5-4"><a href="#cb5-4"></a><span class="pp">#else</span></span>
<span id="cb5-5"><a href="#cb5-5"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Args<span class="op">></span></span>
<span id="cb5-6"><a href="#cb5-6"></a> <span class="kw">using</span> my_format_string <span class="op">=</span> std<span class="op">::</span>string_view;</span>
<span id="cb5-7"><a href="#cb5-7"></a><span class="pp">#endif</span></span>
<span id="cb5-8"><a href="#cb5-8"></a></span>
<span id="cb5-9"><a href="#cb5-9"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Args<span class="op">></span></span>
<span id="cb5-10"><a href="#cb5-10"></a><span class="dt">void</span> log<span class="op">(</span>my_format_string<span class="op"><</span>Args<span class="op">...></span> s, Args<span class="op">&&...</span> args<span class="op">)</span>;</span></code></pre></div>
</blockquote>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="wording"><span class="header-section-number">3</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>In <span>20.20.1 <a href="https://wg21.link/format.syn">[format.syn]</a></span>, replace the exposition-only names <code class="sourceCode cpp"><em>basic-format-string</em></code>, <code class="sourceCode cpp"><em>format-string</em></code>, and <code class="sourceCode cpp"><em>wformat-string</em></code> with the non-exposition-only names <code class="sourceCode cpp">basic_format_string</code>, <code class="sourceCode cpp">format_string</code>, and <code class="sourceCode cpp">wformat_string</code>.</p>
<p>Do the same for their uses in <span>20.20.4 <a href="https://wg21.link/format.fmt.string">[format.fmt.string]</a></span> (renaming the clause to “Class template <code class="sourceCode cpp">basic_format_string</code>”) and <span>20.20.5 <a href="https://wg21.link/format.functions">[format.functions]</a></span>.</p>
<p>In <span>20.20.4 <a href="https://wg21.link/format.fmt.string">[format.fmt.string]</a></span>, the member should still be exposition only. The full subclause should now read (the only changes are the name of the class template, which no longer is exposition only, and the new getter):</p>
<blockquote>
<div>
<div class="sourceCode" id="cb6"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb6-1"><a href="#cb6-1"></a>template<class charT, class... Args></span>
<span id="cb6-2"><a href="#cb6-2"></a>struct basic_format_string {</span>
<span id="cb6-3"><a href="#cb6-3"></a>private:</span>
<span id="cb6-4"><a href="#cb6-4"></a> basic_string_view<charT> <em>str</em>; // exposition only</span>
<span id="cb6-5"><a href="#cb6-5"></a></span>
<span id="cb6-6"><a href="#cb6-6"></a>public:</span>
<span id="cb6-7"><a href="#cb6-7"></a> template<class T> consteval basic_format_string(const T& s);</span>
<span id="cb6-8"><a href="#cb6-8"></a></span>
<span id="cb6-9"><a href="#cb6-9"></a><span class="va">+ constexpr basic_string_view<charT> get() const noexcept { return <em>str</em>; }</span></span>
<span id="cb6-10"><a href="#cb6-10"></a>};</span></code></pre></div>
</div>
<div class="sourceCode" id="cb7"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1"></a><span class="kw">template</span><span class="op"><</span><span class="kw">class</span> T<span class="op">></span> <span class="kw">consteval</span> basic_format_string<span class="op">(</span><span class="kw">const</span> T<span class="op">&</span> s<span class="op">)</span>;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">1</a></span> <em>Constraints</em>: <code class="sourceCode cpp"><span class="kw">const</span> T<span class="op">&</span></code> models <code class="sourceCode cpp">convertible_to<span class="op"><</span>basic_string_view<span class="op"><</span>charT<span class="op">>></span></code></p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">2</a></span> <em>Effects</em>: Direct-non-list-initializes <code class="sourceCode cpp"><em>str</em></code> with <code class="sourceCode cpp">s</code>.</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">3</a></span> <em>Remarks</em>: A call to this function is not a core constant expression ([expr.const]) unless there exist <code class="sourceCode cpp">args</code> of types <code class="sourceCode cpp">Args</code> such that <code class="sourceCode cpp"><em>str</em></code> is a format string for <code class="sourceCode cpp">args</code>.</p>
</blockquote>
<h2 data-number="3.1" id="feature-test-macro-1"><span class="header-section-number">3.1</span> Feature-test macro<a href="#feature-test-macro-1" class="self-link"></a></h2>
<p>Bump the <code class="sourceCode cpp">format</code> feature-test macro in <span>17.3.2 <a href="https://wg21.link/version.syn">[version.syn]</a></span>:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb8"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb8-1"><a href="#cb8-1"></a><span class="st">- #define __cpp_lib_format <span class="diffdel">202110L</span> // also in <format></span></span>
<span id="cb8-2"><a href="#cb8-2"></a><span class="va">+ #define __cpp_lib_format <span class="diffins">2022XXL</span> // also in <format></span></span></code></pre></div>
</div>
</blockquote>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">4</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-P1045R1">
<p>[P1045R1] David Stone. 2019-09-27. constexpr Function Parameters. <br />
<a href="https://wg21.link/p1045r1">https://wg21.link/p1045r1</a></p>
</div>
<div id="ref-P1221R1">
<p>[P1221R1] Jason Rice. 2018-10-03. Parametric Expressions. <br />
<a href="https://wg21.link/p1221r1">https://wg21.link/p1221r1</a></p>
</div>
<div id="ref-P1733R0">
<p>[P1733R0] David Sankel, Daveed Vandevoorde. 2019-06-17. User-friendly and Evolution-friendly Reflection: A Compromise. <br />
<a href="https://wg21.link/p1733r0">https://wg21.link/p1733r0</a></p>
</div>
<div id="ref-P2049R0">
<p>[P2049R0] Andrew Sutton, Wyatt Childers. 2020-01-13. Constraint refinement for special-cased functions. <br />
<a href="https://wg21.link/p2049r0">https://wg21.link/p2049r0</a></p>
</div>
<div id="ref-P2089R0">
<p>[P2089R0] Barry Revzin. 2020-02-17. Function parameter constraints are too fragile. <br />
<a href="https://wg21.link/p2089r0">https://wg21.link/p2089r0</a></p>
</div>
<div id="ref-P2216R3">
<p>[P2216R3] Victor Zverovich. 2021-02-15. std::format improvements. <br />
<a href="https://wg21.link/p2216r3">https://wg21.link/p2216r3</a></p>
</div>
<div id="ref-P2508R0">
<p>[P2508R0] Barry Revzin. 2021-12-17. Exposing std::basic-format-string. <br />
<a href="https://wg21.link/p2508r0">https://wg21.link/p2508r0</a></p>
</div>
<div id="ref-P2508R1">
<p>[P2508R1] Barry Revzin. 2022-01-18. Exposing std::basic-format-string. <br />
<a href="https://wg21.link/p2508r1">https://wg21.link/p2508r1</a></p>
</div>
<div id="ref-std-discussion">
<p>[std-discussion] Joseph Thomson. 2021. Should a <code class="sourceCode cpp">std<span class="op">::</span>basic_format_string</code> be specified? <br />
<a href="https://lists.isocpp.org/std-discussion/2021/12/1526.php">https://lists.isocpp.org/std-discussion/2021/12/1526.php</a></p>
</div>
</div>
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