New issue from S. B. Tam: "time_get::do_get_date is problematic even after LWG 461"

Author: Dani-Hub

xml/issue4285.xml

@@ -0,0 +1,43 @@
+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4285" status="New">
+<title>`time_get::do_get_date` is problematic even after LWG 461</title>
+<section>
+<sref ref="[locale.time.get.members]"/>
+</section>
+<submitter>S. B. Tam</submitter>
+<date>27 Jun 2025</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+Background: <a href="https://github.com/cplusplus/draft/pull/8009">https://github.com/cplusplus/draft/pull/8009</a>
+<p/>
+LWG <iref ref="461"/> changed `time_get::do_get_date` to parse a set of fixed formats. For example, if 
+`time_get::date_order()` is `no_order` or `mdy`, the format parsed by `do_get_date` is `%m/%d/%y`.
+<p/>
+This has several problems:
+</p>
+<ol>
+<li><p>When the resolution of LWG <iref ref="461"/> was applied to the draft standard, the slashes were lost. 
+This could mislead people into implementing `do_get_date` using the incorrect formats. Fortunately, none 
+of the standard library implementations are affected by this mistake.</p></li>
+<li><p>Only 2-digit years are accepted due to the use of `%y`. This could lead to incorrect parse if 
+`strftime` uses `%Y` for the locale's date format.</p></li>
+<li><p>`date_order()` might need additional locale facets to find out the date order, but it doesn't have an 
+<tt>ios_base&amp;</tt> argument from which to get the locale.</p></li>
+<li><p>Many locales do not use any of the specified formats. For example, `de_DE` uses `%d.%m.%Y`, `zh_CN` 
+uses `%Y?%m?%d?`. Although <sref ref="[locale.time.get.virtuals]"/>/5 gives an implementation the latitude 
+to accept additional formats, ambiguity could arise if the locale's format disagrees with `date_order()`.</p></li>
+</ol>
+<p>
+On POSIX systems, it is possible to query the locale's date format using `nl_langinfo(D_FMT)`. 
+Maybe an implementation should be allowed to use that format <b>instead</b> of the one indicated by `date_order()`.
+</p>
+</discussion>
+
+<resolution>
+</resolution>
+
+</issue>