<chrono>

• Change insert()-at-begin() to assign() which is strictly more efficient. This is for a freshly constructed vector where only reserve() has been called, and this isn't in a loop, so the behavior is unchanged. (Note that assign() preserves the reserved capacity when it's sufficient, which is the case here.)
• Remove // TRANSITION: work in progress from tzdb_list (which is complete AFAIK).
• Comment why get_tzdb_list() throws runtime_error instead of bad_alloc for allocation failure.
• Enhance get_tzdb_list()'s exception handling:
  • Use _TRY_BEGIN etc. to preserve our minimal support for _HAS_EXCEPTIONS=0. (We need to directly test #if _HAS_EXCEPTIONS when referring to _Except, because it won't be declared in _HAS_EXCEPTIONS=0 mode.)
  • Add a case to rethrow runtime_error unchanged.
  • In both runtime_error and exception cases, we need to __std_free_crt in order to avoid leaking.
• Similarly, enhance reload_tzdb()'s EH.
• Add _CHRONO qualification to non-_Ugly function calls.
• Enhance tzdb_list's locking. The Standardese talks about multithreading for specific functions or pairs of functions (WG21-N4885 [time.zone.db.list]/3, front() "is thread-safe with respect to reload_tzdb()", [time.zone.db.access]/2 get_tzdb_list() "It is safe to call this function from multiple threads at one time."), and the original implementation appeared to follow this strictly, but I propose to extend this by applying reader/writer locks to all member functions. (Note, for example, that if another thread is simultaneously reloading to emplace_front a new element, neither front() nor begin() can be called simultaneously. The Standard says that front() must be thread-safe in this scenario, so we took a _Shared_lock, but it's silent about begin() which really seems like a defect to me.
  • There are naturally some remaining restrictions (e.g. can't erase_after() while someone is iterating through), but this seems like a cheap way to prevent rare catastrophes.
  • _Shared_lock here is never manually _Unlock()ed, so we can drop that and its bool _Owns to be slightly more efficient.

xtzdb.h, src/tzdb.cpp

• Mark __std_tzdb_get_reg_leap_seconds() as _NODISCARD for consistency and because it allocates.

tests/std/include/timezone_data.hpp

• <string> appeared to be completely unused.

P0355R7_calendars_and_time_zones_clocks/test.cpp

• Include <algorithm> for find().
• Directly construct const sys_days ls{ymd};, avoiding "Almost Always Auto".
• Rename ugly _Tz in test code.

P0355R7_calendars_and_time_zones_io/test.cpp

• As this is C++20, we can use pair CTAD instead of make_pair(), which is simpler (and slightly less debug codegen, not that it matters in test code).

P0355R7_calendars_and_time_zones_time_zones/test.cpp

• Fix comment typos.