<chrono>: Add comments for compiler bug workarounds

stl/inc/chrono

@@ -2209,6 +2209,7 @@ namespace chrono {
             auto [_Icu_version, _Zones, _Links] = _Tzdb_generate_time_zones();
             auto [_Leap_sec, _All_ls_positive]  = _Tzdb_generate_leap_seconds(0);
             auto _Version                       = _Icu_version + "." + _STD to_string(_Leap_sec.size());
+            // TRANSITION, VSO-2228186, should call emplace_front with construction arguments
             _Tzdb_list.emplace_front(tzdb{
                 _STD move(_Version), _STD move(_Zones), _STD move(_Links), _STD move(_Leap_sec), _All_ls_positive});
         }
@@ -2241,12 +2242,6 @@ namespace chrono {
             return _Tzdb_list.cend(); // no lock necessary for forward_list::cend()
         }
 
-        template <class... _ArgsTy>
-        void _Emplace_front(_ArgsTy&&... _Args) {
-            _Unique_lock _Lk(_Tzdb_mutex);
-            _Tzdb_list.emplace_front(_STD forward<_ArgsTy>(_Args)...);
-        }
-
         const tzdb& _Reload() {
             _Unique_lock _Lk(_Tzdb_mutex);
             auto [_Leap_sec, _All_ls_positive] = _Tzdb_generate_leap_seconds(_Tzdb_list.front().leap_seconds.size());
@@ -2266,6 +2261,7 @@ namespace chrono {
                 }
 
                 auto _Version = _Tzdb_update_version(_Tzdb.version, _Leap_sec.size());
+                // TRANSITION, VSO-2228186, should call emplace_front with construction arguments
                 _Tzdb_list.emplace_front(tzdb{
                     _STD move(_Version), _STD move(_Zones), _STD move(_Links), _STD move(_Leap_sec), _All_ls_positive});
             }

tests/std/include/timezone_data.hpp

@@ -170,3 +170,54 @@ void run_tz_test(TestFunction test_function) {
         assert(false);
     }
 }
+
+namespace tzdb_list_hack { // non-portable hack into tzdb_list
+    template <auto>
+    struct member_hacker;
+
+    template <auto Val>
+    consteval auto get_hacked_memptr(member_hacker<Val>) noexcept {
+        return Val;
+    }
+
+    template <>
+    struct member_hacker<&tzdb_list::_Tzdb_list> {
+        friend consteval auto get_tzdb_list_inner_forward_list() noexcept {
+            return get_hacked_memptr(member_hacker{});
+        }
+    };
+
+    template <>
+    struct member_hacker<&tzdb_list::_Tzdb_mutex> {
+        friend consteval auto get_tzdb_list_inner_smtx() noexcept {
+            return get_hacked_memptr(member_hacker{});
+        }
+    };
+
+    consteval auto get_tzdb_list_inner_forward_list() noexcept;
+    consteval auto get_tzdb_list_inner_smtx() noexcept;
+
+    template <class... ArgsTypes>
+    void emplace_front(tzdb_list& tl, ArgsTypes&&... args) {
+        struct [[nodiscard]] unique_smtx_lock {
+            explicit unique_smtx_lock(_Smtx_t& mtx) : mtx_{&mtx} {
+                _Smtx_lock_exclusive(mtx_);
+            }
+
+            unique_smtx_lock(const unique_smtx_lock&)            = delete;
+            unique_smtx_lock& operator=(const unique_smtx_lock&) = delete;
+
+            ~unique_smtx_lock() {
+                _Smtx_unlock_exclusive(mtx_);
+            }
+
+            _Smtx_t* mtx_;
+        };
+
+        constexpr auto memptr_fwdlist = get_tzdb_list_inner_forward_list();
+        constexpr auto memptr_smtx    = get_tzdb_list_inner_smtx();
+
+        unique_smtx_lock guard_{tl.*memptr_smtx};
+        (tl.*memptr_fwdlist).emplace_front(forward<ArgsTypes>(args)...);
+    }
+} // namespace tzdb_list_hack

tests/std/tests/P0355R7_calendars_and_time_zones_clocks/test.cpp

@@ -431,7 +431,7 @@ void test() {
         leap_vec.emplace_back(sys_days{1d / January / 2020y}, false, leap_vec.back()._Elapsed());
         leap_vec.emplace_back(sys_days{1d / January / 2021y}, true, leap_vec.back()._Elapsed());
         my_tzdb._All_ls_positive = false;
-        get_tzdb_list()._Emplace_front(move(my_tzdb));
+        tzdb_list_hack::emplace_front(get_tzdb_list(), move(my_tzdb));
     }
 
     offset = 0s;
@@ -458,7 +458,7 @@ void test() {
             leap_vec.emplace_back(sys_days{1d / January / year{i + 2020}}, false, leap_vec.back()._Elapsed());
         }
         leap_vec.emplace_back(sys_days{1d / January / 2060y}, true, leap_vec.back()._Elapsed());
-        get_tzdb_list()._Emplace_front(move(my_tzdb));
+        tzdb_list_hack::emplace_front(get_tzdb_list(), move(my_tzdb));
     }
 
     offset = 0s;

tests/std/tests/P0355R7_calendars_and_time_zones_io/test.cpp

@@ -1070,7 +1070,7 @@ void parse_timepoints() {
         leap_vec.emplace_back(sys_days{1d / January / 2020y}, false, leap_vec.back()._Elapsed());
         leap_vec.emplace_back(sys_days{1d / January / 2022y}, true, leap_vec.back()._Elapsed());
         my_tzdb._All_ls_positive = false;
-        get_tzdb_list()._Emplace_front(move(my_tzdb));
+        tzdb_list_hack::emplace_front(get_tzdb_list(), move(my_tzdb));
     }
 
     utc_seconds ut_ref = utc_clock::from_sys(sys_days{1d / July / 1972y}) - 1s; // leap second insertion