From e25a2601aaa95c861af3228982ecbb2b1c19d27a Mon Sep 17 00:00:00 2001
From: Marek Kurdej <marek@quasardb.net>
Date: Wed, 13 May 2020 11:37:58 +0200
Subject: [PATCH] [libc++] [LWG3321] Mark "year_month_day_last::day()
 specification does not cover !ok() values" issue as "Nothing to do", but add
 assertion.

Summary:
This LWG issue states that the result of `year_month_day_last::day()` is implementation defined if `ok()` is `false`.
However, from user perspective, calling `day()` in this situation will lead to a (possibly difficult to find) crash.
Hence, I have added an assertion to warn user at least when assertions are enabled.
I am however not aware of the libc++ stand on the desired behaviour.

Reviewers: ldionne, mclow.lists, EricWF, #libc

Reviewed By: ldionne, #libc

Subscribers: christof, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D70346
---
 libcxx/include/chrono        | 2 ++
 libcxx/www/cxx2a_status.html | 2 +-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/libcxx/include/chrono b/libcxx/include/chrono
index 6e5de39..9913dc1 100644
--- a/libcxx/include/chrono
+++ b/libcxx/include/chrono
@@ -824,6 +824,7 @@ constexpr chrono::year                                  operator ""y(unsigned lo
 */
 
 #include <__config>
+#include <__debug>
 #include <ctime>
 #include <type_traits>
 #include <ratio>
@@ -2454,6 +2455,7 @@ chrono::day year_month_day_last::day() const noexcept
         chrono::day(31), chrono::day(31), chrono::day(30),
         chrono::day(31), chrono::day(30), chrono::day(31)
     };
+    _LIBCPP_ASSERT(ok(), "year_month_day_last::day(): year_month_day_last is invalid");
     return month() != February || !__y.is_leap() ?
         __d[static_cast<unsigned>(month()) - 1] : chrono::day{29};
 }
diff --git a/libcxx/www/cxx2a_status.html b/libcxx/www/cxx2a_status.html
index 4b715e7..bee06e6 100644
--- a/libcxx/www/cxx2a_status.html
+++ b/libcxx/www/cxx2a_status.html
@@ -439,7 +439,7 @@
 	<tr><td><a href="https://wg21.link/LWG3209">3209</a></td><td>Expression in <tt>year::ok()</tt> returns clause is ill-formed</td><td>Cologne</td><td>Complete</td></tr>
 
 	<tr><td></td><td></td><td></td><td></td></tr>
-	<tr><td><a href="https://wg21.link/LWG3231">3231</a></td><td><tt>year_month_day_last::day</tt> specification does not cover <tt>!ok()</tt> values</td><td>Belfast</td><td></td></tr>
+	<tr><td><a href="https://wg21.link/LWG3231">3231</a></td><td><tt>year_month_day_last::day</tt> specification does not cover <tt>!ok()</tt> values</td><td>Belfast</td><td><i>Nothing to do</i></td></tr>
 	<tr><td><a href="https://wg21.link/LWG3225">3225</a></td><td><tt>zoned_time</tt> converting constructor shall not be <tt>noexcept</tt></td><td>Belfast</td><td></td></tr>
 	<tr><td><a href="https://wg21.link/LWG3190">3190</a></td><td><tt>std::allocator::allocate</tt> sometimes returns too little storage</td><td>Belfast</td><td></td></tr>
 	<tr><td><a href="https://wg21.link/LWG3218">3218</a></td><td>Modifier for <tt>%d</tt> parse flag does not match POSIX and <tt>format</tt> specification</td><td>Belfast</td><td></td></tr>
-- 
2.7.4