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‎issue3806.html‎

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@@ -67,6 +67,7 @@ <h3 id="3806"><a href="lwg-closed.html#3806">3806</a>. Should concept <code>form
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<b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2022-10-28 <b>Last modified:</b> 2023-01-24</p>
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<p><b>Priority: </b>2
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</p>
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<p><b>View other</b> <a href="lwg-index-open.html#format.formattable">active issues</a> in [format.formattable].</p>
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<p><b>View all other</b> <a href="lwg-index.html#format.formattable">issues</a> in [format.formattable].</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
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<p><b>Discussion:</b></p>

‎issue3925.html‎

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@@ -67,6 +67,7 @@ <h3 id="3925"><a href="lwg-defects.html#3925">3925</a>. Concept <code>formattabl
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<b>Submitter:</b> Mark de Wever <b>Opened:</b> 2023-04-16 <b>Last modified:</b> 2023-11-22</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View other</b> <a href="lwg-index-open.html#format.formattable">active issues</a> in [format.formattable].</p>
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<p><b>View all other</b> <a href="lwg-index.html#format.formattable">issues</a> in [format.formattable].</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
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<p><b>Discussion:</b></p>

‎issue3943.html‎

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@@ -67,6 +67,7 @@ <h3 id="3943"><a href="lwg-active.html#3943">3943</a>. Clarify lifetime requirem
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<b>Submitter:</b> Mark de Wever <b>Opened:</b> 2023-06-01 <b>Last modified:</b> 2023-06-08</p>
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<p><b>Priority: </b>3
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</p>
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<p><b>View other</b> <a href="lwg-index-open.html#format.formattable">active issues</a> in [format.formattable].</p>
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<p><b>View all other</b> <a href="lwg-index.html#format.formattable">issues</a> in [format.formattable].</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>

‎issue4240.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 4240: The formattable type is not a formattable type</title>
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<meta property="og:title" content="Issue 4240: The formattable type is not a formattable type">
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<meta property="og:description" content="C++ library issue. Status: New">
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<hr>
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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="4240"><a href="lwg-active.html#4240">4240</a>. The formattable type is not a <code>formattable</code> type</h3>
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<p><b>Section:</b> 28.5.6.3 <a href="https://wg21.link/format.formattable">[format.formattable]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Hewill Kang <b>Opened:</b> 2025-04-06 <b>Last modified:</b> 2025-04-06</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View other</b> <a href="lwg-index-open.html#format.formattable">active issues</a> in [format.formattable].</p>
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<p><b>View all other</b> <a href="lwg-index.html#format.formattable">issues</a> in [format.formattable].</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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User-specific <code>formatter</code>s usually have the following form:
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</p>
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<blockquote><pre>
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template &lt;&gt; struct std::formatter&lt;T&gt; {
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constexpr auto parse(format_parse_context&amp; ctx)
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-> format_parse_context::iterator;
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auto format(const T&amp; value, format_context&amp; ctx) const
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-> format_context::iterator;
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};
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</pre></blockquote>
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<p>
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This is reflected in wording examples such as 28.5.6.4 <a href="https://wg21.link/format.formatter.spec">[format.formatter.spec]</a> bullet 8 or
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28.5.6.7 <a href="https://wg21.link/format.context">[format.context]</a> bullet 9:
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</p>
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<blockquote><pre>
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#include &lt;format&gt;
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#include &lt;string&gt;
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enum color { red, green, blue };
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const char* color_names[] = { "red", "green", "blue" };
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template&lt;&gt; struct std::formatter&lt;color&gt; : std::formatter&lt;const char*&gt; {
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auto format(color c, format_context&amp; ctx) const {
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return formatter&lt;const char*&gt;::format(color_names[c], ctx);
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}
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};
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</pre></blockquote>
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<p>
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which allows us to format <code>color</code> with <code>std::format("{}", red)</code>.
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Unfortunately, even so, the <code>color</code> still does not satisfy <code>std::formattable</code>.
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<p/>
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This is because the concept <code>formattable</code> is currently defined as follows:
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</p>
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<blockquote><pre>
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template&lt;class T, class Context,
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class Formatter = typename Context::template formatter_type&lt;remove_const_t&lt;T&gt;&gt;&gt;
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concept <i>formattable-with</i> = // <i>exposition only</i>
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semiregular&lt;Formatter&gt; &amp;&amp;
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requires(Formatter&amp; f, const Formatter&amp; cf, T&amp;&amp; t, Context fc,
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basic_format_parse_context&lt;typename Context::char_type&gt; pc)
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{
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{ f.parse(pc) } -> same_as&lt;typename decltype(pc)::iterator&gt;;
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{ cf.format(t, fc) } -> same_as&lt;typename Context::iterator&gt;;
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};
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template&lt;class T, class charT&gt;
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concept formattable =
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<i>formattable-with</i>&lt;remove_reference_t&lt;T&gt;, basic_format_context&lt;<i>fmt-iter-for</i>&lt;charT&gt;, charT&gt;&gt;;
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</pre></blockquote>
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<p>
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where <code><i>fmt-iter-for</i>&lt;charT&gt;</code> is an unspecified type that can write
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<code>charT</code>, which for <code>char</code> is <code>back_insert_iterator&lt;string&gt;</code>
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and <code>char*</code> in libstdc++ and libc++, respectively.
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</p>
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<p>
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That is, for <code>color</code> to satisfy <code>formattable</code>, it is
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necessary to ensure that <code>cf.format(t, fc)</code> is well-formed.
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</p>
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<p>
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However, the <code>format()</code> function in the above example takes a <code>format_context</code>
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whose <code>Out</code> parameter is internal iterator type, namely
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<code>_Sink_iter&lt;char&gt;</code> and <code>__format::__output_buffer&lt;char&gt;</code> in
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libstdc++ and libc++, respectively.
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Since <code>basic_format_context</code> with different <code>Out</code> parameters cannot be converted to
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each other, the constraint is not satisfied.
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</p>
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<p>
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The reason <code>color</code> can still be formatted is that <code>basic_format_arg</code>
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checks for <code><i>formattable-with</i>&lt;Context&gt;</code> where <code>Context</code>
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has been correctly specified as <code>format_context</code>.
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</p>
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<p>
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And since <code>color</code> is formattable but not <code>formattable</code>, this further
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prevents formatting a range with elements of <code>color</code>, because the <code>formatter</code>
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specialization for ranges requires that the element type must be <code>formattable</code>.
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This leads to some inconsistencies (<a href="https://godbolt.org/z/Y8a6WTrK1">demo</a>):
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</p>
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<blockquote><pre>
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std::println("{}", red); // ok
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static_assert(std::formattable&lt;color, char&gt;); // <span style="color:#C80000;font-weight:bold">fires</span>
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std::vector&lt;color&gt; v;
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std::println("{}", v); // <span style="color:#C80000;font-weight:bold">not ok</span>
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</pre></blockquote>
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<p>
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The workaround is to turn the custom <code>format()</code> into a template function
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such as <code>format(color c, auto&amp; ctx)</code> or
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<code>format(color c, basic_format_context&lt;Out, charT&gt;&amp; ctx)</code>,
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However, this seems mandate users to always declare <code>format()</code> as the template
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function for the best practice, which in my opinion defeats the purpose of introducing
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<code>format_context</code> in the first place.
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</p>
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<p>
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Also, since <code><i>fmt-iter-for</i>&lt;charT&gt;</code> is unspecified, if it is specified
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in some library implementation as the same type as <code>format_context</code>'s <code>Out</code>
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parameters, then <code>color</code> will suddenly become <code>formattable</code>. This lack
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of guarantee about <code>formattable</code> can bring unnecessary confusion.
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</p>
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<p>
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I think we should ensure that <code>color</code> is <code>formattable</code>, because it is formattable.
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</p>
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<p id="res-4240"><b>Proposed resolution:</b></p>
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<p>
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This wording is relative to <a href="https://wg21.link/N5008">N5008</a>.
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</p>
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<ol>
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<li><p>Modify 28.5.6.3 <a href="https://wg21.link/format.formattable">[format.formattable]</a> as indicated:</p>
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<blockquote>
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<p>
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-1- <del>Let <code><i>fmt-iter-for</i>&lt;charT&gt;</code> be an unspecified type that models
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<code>output_iterator&lt;const charT&amp;&gt;</code> (24.3.4.10 <a href="https://wg21.link/iterator.concept.output">[iterator.concept.output]</a>)</del>.
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</p>
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<blockquote><pre>
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[&hellip;]
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template&lt;class T, class charT&gt;
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concept formattable =
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<i>formattable-with</i>&lt;remove_reference_t&lt;T&gt;, <ins>conditional_t&lt;same_as&lt;charT, char&gt;, format_context, wformat_context&gt;</ins><del>basic_format_context&lt;<i>fmt-iter-for</i>&lt;charT&gt;, charT&gt;</del>&gt;;
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</pre></blockquote>
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</blockquote>
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