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New issue from Jiang An: "The parse function of a BasicFormatter type…
… needs to be constexpr"
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| <?xml version='1.0' encoding='utf-8' standalone='no'?> | ||
| <!DOCTYPE issue SYSTEM "lwg-issue.dtd"> | ||
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| <issue num="3993" status="New"> | ||
| <title>The <tt>parse</tt> function of a <i>BasicFormatter</i> type needs to be <tt>constexpr</tt></title> | ||
| <section><sref ref="[formatter.requirements]"/></section> | ||
| <submitter>Jiang An</submitter> | ||
| <date>10 Oct 2023</date> | ||
| <priority>99</priority> | ||
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| <discussion> | ||
| <p> | ||
| Format strings need to be checkable at compile-time since <paper num="P2216R3"/>. In order to check a | ||
| compile-time format string, the <tt>parse</tt> functions of the formatters need to be called. However, | ||
| currently there is seemingly no requirement for the <tt>parse</tt> function to be <tt>constexpr</tt>, | ||
| and hence whether a format string is correct for formatted types may be not well-defined at compile-time. | ||
| </p> | ||
| </discussion> | ||
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| <resolution> | ||
| <p> | ||
| This wording is relative to <paper num="N4958"/>. | ||
| </p> | ||
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| <ol> | ||
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| <li><p>Modify <sref ref="[formatter.requirements]"/>, Table 73, <i>BasicFormatter</i> requirements [tab:formatter.basic] as indicated:</p> | ||
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| <blockquote> | ||
| <table border="1"> | ||
| <caption>Table 73: <i>BasicFormatter</i> requirements [tab:formatter.basic]</caption> | ||
| <tr> | ||
| <th align="center">Expression</th> | ||
| <th align="center">Return type</th> | ||
| <th align="center">Requirement</th> | ||
| </tr> | ||
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| <tr> | ||
| <td> | ||
| <tt>g.parse(pc)</tt> | ||
| </td> | ||
| <td> | ||
| <tt>PC::iterator</tt> | ||
| </td> | ||
| <td> | ||
| Parses <i>format-spec</i> (<sref ref="[format.string]"/>) for type <tt>T</tt> in the<br/> | ||
| range <tt>[pc.begin(), pc.end())</tt> until the first unmatched<br/> | ||
| character. Throws <tt>format_error</tt> unless<br/> | ||
| the whole range is parsed or the unmatched<br/> | ||
| character is <tt>}</tt>.<br/> | ||
| [<i>Note 1</i>: This allows formatters to emit meaningful<br/> | ||
| error messages. — <i>end note</i>]<br/> | ||
| Stores the parsed format specifiers in <tt>*this</tt> and<br/> | ||
| returns an iterator past the end of the parsed<br/> | ||
| range. | ||
| <ins>The expression shall be a core constant expression if <tt>g</tt>,<br/> | ||
| <tt>pc.begin()</tt>, <tt>pc.end()</tt>, and lvalue-to-rvalue conversion<br/> | ||
| for each element in <tt>[pc.begin(), pc.end())</tt> are core constant<br/> | ||
| expressions, and no exception is thrown.</ins> | ||
| </td> | ||
| </tr> | ||
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| <tr> | ||
| <td colspan="3" align="center"> | ||
| <tt>…</tt> | ||
| </td> | ||
| </tr> | ||
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| </table> | ||
| </blockquote> | ||
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| </li> | ||
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| </ol> | ||
| </resolution> | ||
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| </issue> |