[Bug libstdc++/121154] New: Incorrect output of format(loc, "{:L}", x) for invalid month/weekday and loc not classic.

[tkaminsk at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121154

            Bug ID: 121154
           Summary: Incorrect output of format(loc, "{:L}", x) for invalid
                    month/weekday and loc not classic.
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tkaminsk at gcc dot gnu.org
  Target Milestone: ---

The following code:
  std::locale loc_fr("fr_FR.ISO8859-15");
  std::cout << std::format(loc_fr, "{:L}", std::chrono::month(0)) << std::endl;
Prints "?" instead of "0 is not valid month".