[Bug libstdc++/126543] chrono::parse does not reject "60" seconds for a non-leap second

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126543

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2026-07-31
             Status|UNCONFIRMED                 |NEW

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #0)
> The last assertion fails, because we allow ":60" for any utc_time, but we
> should verify that the parsed time really was a leap second.

Currently the first assertion also fails, but that's easier to fix:

-- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -4778,7 +4778,7 @@ namespace __detail
                                       && !__is_floating)
                    {
                      auto __val = __read_unsigned(__num ? __num : 2);
-                     if (0 <= __val && __val <= 59) [[likely]]
+                     if (0 <= __val && __val <= 60) [[likely]]
                        __s = seconds(__val);
                      else
                        {

We already have code to check for seconds(60) later, causing us to set
_M_is_leap_second, and then that causes us to fail for sys_time, gps_time, and
tai_time.